After over four decades in the fitness biz, I’ve come to believe that the reason someone will or won’t exercise is a little more subtle than just “willpower”. People who would never even think about exercise will suddenly warp their lives around it if the proper incentive is there. A high school reunion, becoming single again and going back on the market…
A very distant second is finding the right type of exercise for a particular individual. I love being in a gym, but don’t do nearly as well exercising outdoors with bodyweight stuff, for example. A distance runner friend once remarked about how funny it was — he would happily go slog through a cold muddy 40k in the rain but found weight training utterly boring. Other people enjoy rowing, or tennis, or swimming.
I view motivating someone to exercise more like picking a lock than trying to inject them with willpower. Older people will often respond to simple self-interest (“Start exercising or you’ll die.”) if they’ve just had a health scare. Young people will get into it if they think that it’ll make them more attractive. You find the right combination and pretty much anyone will do it.