This is certainly true - for some people, running is constitutes a significant net positive (in terms of motivation/rewards).

A few arguments, though:

1. to me, it doesn't seem the case of the OP (if it was, they'd be already doing it)

2. I know only few people who consistently run, so my opinion is that the motivation/energy balance is not positive for the vast majority of the people (at least, from the mid-term onwards)

3. regarding the "exercise" concept, it depends on what one means. exercise as functional-training-at-home-or-in-the-park is hard to carry in the mid/long term; it's terribly boring and I believe only highly motivated people can do it. if one talks about organized sport activities, I'm all for it; it's definitely something that I generally suggest, but my observations is that those who are not already doing one in the 30s/40s, don't find sport fun in general and don't stick to (or even start) one.