I've been participating in triathlons/5ks/10ks since I was in high school. To keep in shape, I have a pretty simple regiment that I do 5/6 days a week. Each one of those days, I run, bike, or swim, depending on what I feel like doing. I go for how ever long I feel appropriate - long enough that I feel tired but not absolutely exhausted. I also go to the gym for half an hour to an hour each of those days. I do a lot of olympic type lifting (snatches, lifts, presses, rows, squats) and resistance training (pushups, situps, pullups, dips). Every now and then I throw something else in just to keep things exciting (like rowing, rock climbing, kayaking, skiing in the winter). I don't consider anything I do very strenuous, definitely something everyone is capable of. It winds up only being about an hour or two every day, which isn't much considering you're awake for about 16.