"Any exercise" gets my vote. It seems to me that a lot of people - myself included - get wrapped up in the body hack mentality (4-Hour Body?) or in finding the "best" exercises, when the important thing is just to get moving and make it fun.
You know, doing anything can actually hurt you more then it helps. For example "just go outside running" can be really bad to your feet and joints, if you don't wear the right shoes/clothes, do it too much, without warming up. It can also be bad for your heart, if you run too fast. "Any exercise" is really the worst thing possible.
The body you were born with? You're allowed to use. No really -- you don't need permission from your doctor, your personal trainer, or the Internet before you do so. Really, this bullshit idea that exercise is something exotic and dangerous doesn't help anyone. Exercise is just doing stuff with your body. It's the most natural thing in the world, the thing that every child knows how to do. You don't need special clothes, special shoes, or people's permission to go and do it.
Regarding running--if you aren't a runner, yes, trying to go out and run five miles might just do more harm than good. Start slowly--run a block, walk a block, run a block--gradually building up to a mile, then two miles.
Most exercise is going to hurt a bit if you've lived a relatively sedentary life. There is a difference between the pain of "my body isn't used to activity" and the pain of "ZOMG, my heart is going to explode." Pain is just weakness leaving your body.
That's true, but I would argue that your body will give clear signals if you're doing something you shouldn't be. Things like sprains, sudden joint pain, and light-headedness have a very clear purpose: to stop you from continuing.
Perhaps I should add "and listen to your body" to my previous comment.
