I think there are a couple of destructive attitudes that exist out there about fitness that get in the way not only of realism but also are missed opportunities for character depth and development.
Attitude One: It's good genes, is all.
A very good friend of mine a very, very long time ago gave my DH a dirty look when he mentioned something about physical fitness and said, to paraphrase, "Well it's easy for you. You've got good genes."
It's never easy, even with good genes. As the years have gone by, my DH and I have to work harder (and eat less) to stay in reasonable shape. And we are not in particularly good shape. I try to look at my workouts as tricks I'm training my body to do, rather than view it as a routine, because routines are boring and never, ever end. For example, the latest trick I'm trying to teach my body is to be able to push up with my hands under my shoulders, flat-backed, from a position where I'm flat against the floor. There's not much leverage from that position. I can do it once if I arch my back a little initially, but that's cheating. After my morning attempt (failure) I usually do some plank exercises in hopes that the next morning, I'll be able to do a real one. And after I've done a real one, I'd like to do, say, five, and then I'll make up some other weird goal for my upper body. For abs, I'm working on a full body belly roll. And for my dex/aerobics, I'm learning how to jump rope again. I used to be pretty good ... when I was eight years old. Now, not so much.
Anyway, even those of us with decent genes can't slack off, so neither can our fictional characters. And many of them don't have gyms to go to, or even a culture that embraces exercise. So either they're going to be engaging in some sort of physical labor, or walking or horseback riding everywhere, or doing some other sort of practical training where there's an obvious pay off.* It's not going to be auto-magic. And by practical, I mean life-saving training where the goal may not necessarily be to look buff, but to stay alive. As the beautiful quote in Zombieland goes, "The first rule of Zombieland is cardio."
Attitude Two: Well, fictional characters have all their off screen time to get into shape and stay in shape, or they can do it with a montage.
Yes, but if it's a daily thing, and there's a lot of variety or a ritualistic quality to it, there's opportunity to show what your characters are made of. Some of them may be the equivalent of gym bunnies and work on their bodies all day, while others could do a short stint of intense exercise and then they're done for the day. Either way, it doesn't have to be some automatic thing to blow off. They should work hard, and be punished or rewarded accordingly. (Don't forget your overuse injuries!) By the way, new research shows that ten minutes of intense exercise a day has the same benefits as hanging out on the treadmill for hours reading a magazine. Sweet! But it has to be intense. So give them an excuse to do something intense every day, or make it part of their training routine, and create some consequences and characteristics that carry over to your story, and see what happens. Three letters. MIB. His fitness and spirit carries through the whole first movie. It's a beautiful thing.
Failing that, consider giving a character an average or even overweight body. Why not? They can still be attractive. In The Ladies' Number One Detective Agency, the main character is 'of tradition build' and is very desirable. We can celebrate health in all different sizes, a full range of capabilities and disabilities in our characters, and have it make sense. Which reminds me, remember, a lot of folks in wheelchairs are athletes too. It's not all about the chair. It's about the person in the chair.
There's a huge variety of awesome humanity out there. If the Phantom of the Opera can be seductive, than anyone can be.
I guess what I'm suggesting, ultimately, is that realism doesn't have to be a drag, any more than real exercise has to be a drag. I love my exercise time (when I get around to it. Ahem.) That's me time. And for those that don't ... you don't have to be Arnold or Demi to be sexy, healthy, happy and effective. Neither do your fictional characters. Give the reality a chance on the page and see what happens, just for fun. There are zillions of sexy long-haired whatevers out there in fiction who get their sleek, fit bodies for free. Dare to be different. Dare to be closer to life, and see what truths you reveal in your story that might not have turned up if you hadn't thought about physicality beyond a description of height and eye color.
And that super-fit guy with the lightning reflexes? There are few things that will gut him faster than not being strong enough to succeed when he's the strongest thing around. Then we'll get to see his real strength, and that's powerful stuff to see on the page, and in real life.
*One of the most painfully laughable scenes I've ever seen on exercise was in Cave Dwellers where the characters had rigged up a weight machine with rough rope, pulleys and rocks. WTF? He's a sword-slinger! You don't think his arms would be strong enough just from that? And he can't afford to be muscle bound. Oh for pity's sake ... but at least they were trying to explain why he looked like a body builder, I guess. Still. Oh. My. Gawd. That was so silly. Conan the Barbarian? Way better. Just saying. Although, it would have made more sense if that wheel wasn't in the middle of nowhere. It would have made more sense if it was in the middle of town or at least near a town and they were grinding wheat or something. Still, better than rock weights. A lot, lot better.
My tweets - - *Mon, 14:56*: an interesting take on the all-white-male photos = it's on purpose https://t.co/enHRH8UBuc - *Mon, 17:55*: Whaaaaaaaaaat?!?!?...
10 hours ago