I used to have the same problem -- NO ENERGY -- until I started lifting weights. The coolest thing about it was that I started feeling more energy immediately. And you can start out with very light weights. You don't need a lot of money to start ... go to the library and find some books about lifting weights. Dumbbells are very cheap -- about 50 cents per pound usually -- and you can often find used ones for even less (Play It Again Sports, if have one near you).
I know that walking has been a miracle for a lot of people, but I was never able to achieve much doing it ... probably a lot of that had to do with developing sore feet and the fact that I had no energy and all the walking just didn't make it any better.
Here's an article I found very interesting, about not being able to do all that jumping around: http://www.stumptuous.com/weights.html
Click on Getting Started, then go to the article titled "No Fat Chicks"