[cko]

I agree that it's not easy to overcome the inertia (the psychological aspects). I've been skinny all my life and I still have to motivate myself to exercise.

You don't necessarily need to "go to the gym" though. There are lots of bodyweight exercises that can produce great results. Not just pushups, either:

http://www.beastskills.com/tutorials/

http://www.dragondoor.com/articles/building-an-olympic-body-...

Some of those exercises are very difficult - the only one I can do is the single leg squat (pistol). But pullups, dips, pistols, sprinting, kettlebells (cheap one-time purchase) can take you a long way.