I really really must agree with Gromnir's post except I've always been of the (possibly daft) opinion that doing nothing is worse than a gently increasing something. I speak as someone who due to possessing a stronger willpower than body has two partially torn biceps, busted knees, and a wedge-shaped dent in my right shin*. Take things slowly and keep in mind that so long as you make a regular thing of exercise there's no practical way you aren't going to feel better in the long term. You've got the whole of your life, so take it easy.
The other thing I'd bring up that Meta already mentioned is stretching and warming up/warming down. These are essential, not optional.
*kickboxing