Yes, a character can train their flexibility and strength even if they don’t have regular access to facilities or a trainer. Important training like conditioning and flexibility can be done solo, and will be an important part of any serious, athletic character even when they do have access to a trainer and appropriate facilities.
Most high schools and colleges have a track, bleachers, and other amenities that are available to the public for use when school is not in session. I’m not talking about the weight lifting gym or anything like that (though if your character is a student, they may be able to take advantage of it by going through the appropriate channels). For liability reasons, the weight lifting gym in a high school, even a public one, will be closed to anyone who isn’t an athlete. But the track? The bleachers? The pullup bars? And other amenities? Those they can use.
If you have any experience doing conditioning training (building stamina, lung capacity, etc) then use that. If you don’t and I’m guessing this is true, we’ll go over some simple training exercises below and talk about the problems with pushing the boundaries of believability.
Now, when working with conditioning, it’s important to not over exaggerate. In many instances when I’ve been reading, I’ve found authors who didn’t have much experience with training pushing themselves either too high or too low. For example, in the scope of full out physical exertion five minutes is a long goddamn time. A usual workout for a character is going to only last between fifteen to thirty minutes, not an hour. They’re going to need to take breaks between one minute to five minutes and if they’re alone then they’ll have to moderate that for themselves. The length of the break will change based on the amount of exertion, say if they’ve been running bleachers, or a mile, or wind sprints, then take five once the repetitions are complete. If they’ve been doing pushups or situps, then a one minute break for some water is applicable.
They will probably feel the desire to cheat, if they do that’s okay just make sure it comes back to bite them later, working out is very hard. Characters who aren’t used to working out will slack off when there’s no one there to watch them. When someone is held accountable only to themselves then things tend to slip. It’s good to have a workout buddy, someone who will push the character forward past their self-perceived limits, but those are also hard to come by.
When doing conditioning, count it out not by time spent but in number of repetitions or reps. Doing pushups for a full minute (fifty/sixty pushups for sixty seconds) is not a beginner sport, serious athletes will do it, but it’s difficult. When working, keep it simple. 5 repetitions of 10 is good for someone who is very experienced whether that’s pushups, situps, leg lifts, or any of the vast number of other exercises out there. When you break the number out, it means they did 50 of each. You can stretch and do reps anywhere, on the bench at school, in front of the television, it doesn’t matter. The place doesn’t need to be special, what is important is that your character is doing them.
Here’s the average layout of the workout we used to do in our Saturday Morning Trainings, these trainings usually lasted between 6:00AM to 7:00AM:
5:30AM to 6:00AM: run a warm up lap before the instructor arrives and stretch.
6:00AM: run a mile (mile will last between 6-14 minutes depending on student and the student’s conditioning, the faster you run, the longer your break)
6:15AM: Practice forms or stances around the track.
6:20AM: Wind sprints/Run a “Korean” Mile (this is what we called it, but line everyone up in a line and send them jogging, last person in line sprints to the front, then over, and over, until the mile (usually for us just a lap or two around the track) is complete. The less teamwork, the harder it is for everyone. The team must slow down to keep pace with their slower members or the line gets really long.)
6:30AM: Practice kicks. (On the chain link fence, we spread out, and practiced our kicks as the instructor counted out the numbers of 1-5. 1: beginning position, 2: chamber, 3: kick, hold kick, 4: chamber, 5: drop the leg. Position changes when he speaks, so you could hold the leg there for a long time.)
6:45AM: Run bleachers. Students younger than twelve or thirteen run the stairs between the bleachers, teens and adults run the actual ones. Count out 5 repetitions, pair the children closer together so that they race.
6:50AM: wind down/cool off. Pushups, situps, and leg lifts. 2-5 repetitions of 10, depending. (2 for pushups and leg lifts, 5 for situps if time allows).
7:00AM: stretch. Everybody goes home.
As for stretching, you don’t need someone else there to help your character stretch. My advice: go to your local bookstore or library and buy or check out a book that’s dedicated to teaching someone how to work out on their own. It will cover all the major pitfalls and missteps a beginner will have, while also helping you add a sense of realism.
You also might want to think about starting to work out for yourself. The actions alone won’t be enough to convey the feelings or mental stress of working out. We can’t really fake remembering the feeling of a runner’s high, personal experience will make you a better writer in the long run.
And please, never ever use second hand training gear that doesn’t fit you. It won’t work and is more dangerous in the long run, it will also hamper your character’s ability to perform and is more likely to get them injured. Now, most workout/weightlifting machines can be adjusted to someone’s personal settings. So, it’s not the end of the world.
Figure out what your character is training to do, then develop a routine that will develop those aspects of their body’s muscular structure. All the training in the world won’t help if they’re developing their body to do the wrong things. All training and all workouts are not created equal.
I hope that helps.