I wrote the following some time ago, I'll comment on what I've written below.

Exercise is unlike almost any other activity you might choose to spend your time on. A recent major study pooling data on 650,000 people observed over 10 years showed that for every hour of moderate exercise you could gain 7 hours or possibly even more of extra life expectancy. When you spend time working out you’re not spending time, you’re gaining time! Spend some quality fun time working out and gain 6 times that time to enjoy other activities.
We all know also that regular exercise also improves the quality of that longer life:
You’ll find your mood improves.
You can more easily control your weight.
You build up and strengthen :
muscle, the healthy tissue of your body
even bones
You increase your energy levels.
You reduce your risk of chronic disease.
Your skin will be healthier.
Your brain health and memory improve.
Your sleep will improve and you’ll be more able to relax.
Why don’t more people do hard physical work in the gym? Our bodies are designed to tell us to save energy, sit on the couch, and not to waste valuable energy by working out. For most of human history work was something you had to do to gather food and create shelter. The body still needs exercise but our bodies will tell us it is better to sit on the couch. Your body however WILL thank you for fitting some vigorous activity, or maybe some long slow aerobic type activities, into your routine.

I think the above is probably all true. But it is also true that behavioural genetics show that variation in weight in the human population is 70% heritable. I think what we should understand from this is that some of us are in the situation that our natural talents and tendencies make putting our time and effort into working out seem worthwhile and make it something we can stick to over time. I'll write more about the findings of behavioural genetics another time. The get fit thing is an interesting paradox I think, just as the diet thing is.