Walking. Long daily walks.
It changed (& helped save) my life.
Edit (to give some context):
When I started to walk, I was barely able to walk at all. Like, really, a few steps to get to the mailbox would kill me and have me lay on bed for hours. I was in a really bad, bad shape (in the head too). Nowadays, I will walk 8-10km every single day and, added to that, I will go everywhere walking if at all possible. I'm still not an athlete but at the least my body is not a dead weight anymore (I feel better in the head too). And it all changed the day I decided I would simply walk a little more. A few steps at first, and then more, and then more. I was impressed by the huge impact of a seemingly little change. I celebrated each 'win' (the first time I walked the block, the first kilometer, and so on) and I never blamed myself for the (many) fails. Instead, I tried to analyze the reasons why I failed so I could do better next time.