It can be hard to judge how much exercise you should do. I echo Eowyn regarding walking, I think it's the safest way to get moving. I also built my stamina up from shuffling around the block and collapsing in the armchair to yesterday walking 5 1/2 miles up hill and down dale and feeling great afterwards!

Early on I recall one time walking too far and realising I had to walk back, by the time I got back I was having difficulty standing, felt spaced out and had to rest for an hour thinking I'd done some damage but the next day I actually felt better.

I think there must be a benefit both psychologically and physically to getting out and about, I'm sure working the muscles, improving the blood flow to the brain and other organs can only help. But there are times you should rest and take it easy too if you feel tired.