What ever you decide to do start off slowly and gradually building up how much you do over a few weeks.
On your treadmill start off at a walking pace and build this up over a few weeks and then increase the speed that you are walking. You should still be able to hold a conversation while you are walking so my instructor used to say.
You have to do something you enjoy the more variety you put in to it the better. If you did the same thing every day every week for months you would get bored. Just doing 20 mins a day is better than doing nothing.
I have to start all over again in 2009 as I cracked my ribs in July and they are only just healed (although this phlemy cough has caused problems with them again).
I loved boxing as it was a great stress reliever and now have to fine something less energetic to start off with. So I am going to do some of my exercise DVD's.
Good luck with what ever you decide to do. Housework around the house to loud music is still good exercise as well Mrs Twiddle.