A/N: I thought about trying to incorporate this into a different story but decided to just post it as a stand-alone oneshot. No spoilers for season 6.

"Come on, Castle. You can do it."

She spoke so easily, as though she were not winded at all from the exertion as they rounded the bend in the park.

"Woman... you are trying... to kill me."

Castle considered himself to be in pretty decent shape. At least, he'd believed that before he'd taken Kate up on her offer to go running together. After all, he stayed fairly active. He had a gym membership which he... sometimes... used. He did prefer the weight lifting machines rather than cardio exercises. It just seemed more manly to pick up heavy things than to run around a track or peddle a stationary bike. And the zumba classes... He wasn't sure he could live it down if some fan snapped a photo and posted it online.

But running shouldn't be so difficult. It shouldn't leave him feeling like his lungs were going to burst out of his chest like a parasitic alien.

"Just a little further," Kate encouraged him. He could make out the amused grin she shot him as she slowed her speed to match his flagging pace.

Why were they doing this? Castle wondered to himself. Why was he subjecting himself to not only physical torture, but why was he letting Kate witness his haggard breaths and struggle to keep up with her?

Oh yeah, it was his big mouth.

A week earlier, it began with a bit of bravado in the bedroom. After a particularly... active evening of bedroom pleasantries, he distinctly remembered bandying about the words "stamina" and "vigor" during post-sex pillow talk. Kate had giggled at him, flashing her megawatt smile as she let him dig himself deeper and deeper.

"So you think you can keep up with me, huh, Castle?" she asked innocently enough.

He'd thought she'd meant... well, something else. Something more inclined towards candles, whipped cream, and perhaps some creative uses for her police cuffs. Castle had no idea she was leading him down a path towards running shoes and a jog through Central Park.

"Absolutely," he'd boasted. And that sealed the deal. Later, when she issued her challenge, he could not very well back down. He would look weak or... unmanly.

Of course, he felt silly as he puffed along behind her at a slow jog, the sweat running down his forehead threatening to blind him. Weren't men supposed to be better runners than women? Something about women having wider hips... Well, that assumption was blown, especially as it applied to him and Kate.

After he motion for her not to wait for him, she had jogged on ahead at her own, much faster pace to the end point of their run and was already circling back to collect him. Knowing that the finish was in sight, Castle pushed himself with a burst of speed which took him out of his slow pace into an excruciating, slightly less slow pace.

"Good job, Castle," Kate praised him as she pivoted to join him on his last leg of the run.

Unable to answer her, he just pressed on ahead, forgoing the necessary oxygen in his lungs in order to impress her. By the time he reached the end, spots were beginning to appear in his vision and he worried that the heat of the day might be taking too great a toll. Passing the tree which marked the end of their circuit through the park, Castle slowed to a gentle walk, ignoring the urge to simply pass out on the grass.

"You did really good," Kate told him, and he ignored her apparent lack of fatigue after the run. "Three miles is really hard to do, especially when you haven't been running regularly."

Or at all, Castle mentally added. He had always known Kate was in phenomenal shape. And while he had a good idea of the effort it took to remain that way - early morning runs, hours spent at the precinct gym, and a healthy diet - Castle had not ever realized how hard it was to maintain.

"How far do you... normally run?" he asked between breaths, his heart still pounding in his ears but his vision having cleared.

"On weekdays, maybe five miles, or whatever I have time for. On the weekends, I try to go a little further."

Five miles? So they hadn't even run as far as she normally ran on a weekday, let alone on a weekend. She had likely picked the distance as something she knew would be difficult for him, but not impossible. While Castle's pride hurt almost as much as his aching legs and burning lungs, he was pleased that she hadn't issued a challenge which would have utterly destroyed all sense of his masculinity.

"But I've been doing this for years, Castle. You can't expect to just go out and do a marathon without training for it."

Kate had a point. Practice makes perfect, as he'd always told Alexis when she'd been growing up and became frustrated with her inability to master an activity the first time she tried it.

"Have you ever run a marathon?" Castle asked, suddenly curious whether she'd ever done one of those 26.2 mile races. While he knew that Kate was active, that she had to be for her job, that part of her life had always been largely invisible to him. Sometimes he found her washing up after a run or doing yoga, but somehow he rarely actually saw her exercising.

"Yeah, I've done two," she answered. They continued along the path in Central Park at a more sedate walk which allowed Castle to recover his normal breathing. "I did one when I was in college, and then I did another one a few years ago to prove to myself that I still could."

"Wow," Castle said, clearly impressed. "Now that's stamina."

Kate shrugged offhandedly. "It's not that big of a deal. Running's just exercise, and sometimes it's a way to get out of my own head."

He nodded in understanding. In some ways, Kate was something of an introvert. She enjoyed her solitary time while he began climbing the walls if left alone too long. And running was a one-person activity. While several people might run together, as she said, it was very much about being in one's own head.

Castle, on the other hand, preferred exercising with others. And he preferred being active in ways that did not actually reek of pure exercise.

"Well, now that we've done your sport," he said, sounding slightly smug, "now we can do my favorite sport."

Kate smiled, skeptically amused. "And what sport is that?" she asked.

"You'll see."

An hour later, Kate found herself in Castle's semi-dark bedroom bending and twisting, lunging and parrying with as much skill as she could muster. And it was hard.

"I didn't know you could actually fight with these things," she muttered, half to herself as she countered a blow from Castle. An eerily accurate fizzling sound erupted from her weapon.

"Oh yeah," he answered. "I made sure to get the extra heavy duty kind just so Alexis and I could fight with them."

Their lightsabers met once more as he pushed an attack and Kate countered it. Her blade glowed red in the low light while his burned in a classic blue. The noises sounded so realistic that she sometimes found herself thinking they really were laser swords.

Suddenly Kate stopped. "Tell me these aren't Alexis' two thousand dollar collector's edition lightsabers..."

Castle waved her off. "No, no. We only use those for special occasions. These are just for practice."

Kate smirked at the thought of "special occasion" lightsabers versus "everyday practice" ones. Only Castle.

Despite the distraction of the light and the noise, Kate found the constant action of swinging and blocking and dodging to be rather invigorating. All geekiness aside, it was very good exercise.

"You're pretty good at this," she observed as he pivoted their blades to rake the tip of his lightsaber across her bicep. Kate knew from the movies that if the weapons were real, she would have lost the arm. Her few strikes against Castle would have only annoyed his tailor.

"I should be. Alexis and I used to practice every day after school until she started doing fencing."

They continued on in a similar manner for a while longer until Kate had to take a break to get some water. She found herself even more worn out and sweaty than after a typical five mile run. The constant motion of moving her arm had also begun to fatigue her muscles.

After chugging down a glass of ice cold water, Kate asked, "Did you learn your technique from watching the movies or...?"

"Some of it is from the movies. Of course, the movies had a lot more flips and things. I tried some when Alexis was much younger, and I almost threw my back out. But a lot of the techniques I use are actually historical. Of course, you can't block a lightsaber with your hand."

Kate eyed him a moment before deducing, "Historical, huh? That means you learned them from watching The Princess Bride, didn't you?"

"Those fight scenes were very accurate!" Castle protested.

She laughed at his earnestness and enjoyed the wrinkles of amusement she saw around his eyes even as he feigned a pout. In many ways, Castle pretended to be less competent than she knew him to be. Part of it was his comedic, self-deprecating nature. But she suspected that he also did not give himself enough credit.

"Well I have to admit, Castle, lightsaber fighting is very good exercise. We should do this more often."

"And it's fun!" he added gleefully.

She returned his kid-like grin. "The sound effects definitely add to the experience," she agreed.

"Oh, for our next bout, I should put on the soundtrack from The Empire Strikes Back. Oh, we can reenact the scene from Cloud City where Luke Skywalker and Darth Vader duel." He stopped as though to seriously think about the logistics of such a re-creation. "I suppose to have the right feel to it, we should go up to the roof this evening, when it gets dark..."

Kate was already shaking her head. "We're not fighting on the roof, Castle."

"But I have the blue lightsaber. You have the red one, so get to be Darth Vader, which isn't a bad role for you. But since I have the blue one, I'd be Luke and I'd be the one to pretend to fall."

She continued to shake her head even more vigorously. "No roofs, Castle."

She hadn't told him much about that day with Maddox when she'd almost fallen to her death and she had no desire to kill their playful moment by explaining her reluctance. Luckily, Castle simply accepted her reproach and agreed, "Okay, then we'll have to substitute the stairs. But you have to get the quote right if you're going to play Darth Vader. Everyone always thinks he says, 'Luke, I am your father,' but that isn't exactly how it goes..."

Kate continued to half-listen to Castle's discussion on the nuances of The Empire Strikes Back as he led her back to the bedroom for round two of their lightsaber duel. She could not help but admire the man. Despite all his fame and fortune, he still enjoyed the simple pleasures in life like playing with what were essentially toy swords. And he did not hide his interests from her. Rather, he reveled in her willingness to play with him.

And she considered the fact that he had shared with her one of his favorite pastimes, something he did with his daughter. It was probably a lot like their laser tag games, she realized, a father-daughter tradition.

"Castle..." she said, stopping him in his tracks.

He looked at her curiously with just a hint of trepidation. "Yeah?"

"Thank you."

"For what?" he asked.

She thought about him jogging with her through the park despite not being much of a runner. But beyond his desire to do things with her, she appreciated his passion to simply do things, to try out new experiences, never settling for the status quo.

"For being you," she settled on.

A/N: As a side note, I recommend Ultra Sabers for all your combat lightsaber needs.