Any exercise is good provided you actually get on and do it. As everyone says walking is good, at least 30 mins and at a pace where you are breahting hard but still able to hold a conversation. This may seem difficult at first if you have not exer ised for a long time but it quickly becomes easier, once it does try adding in some other exercise, or having a couple of two minute bursts of speed walking in your normal walks ( just speed up enough to raise your heart rate and perhaps break a sweat or find conversation difficult).
It can actually become quite addictive. I started exercising again back in April this year, walking and using the rower initially, then quickly adding in cycling and kayaking, and now I do weight training too. I'd love to do some running and go back to climbing too but I still have a worn out knee joint so I need to do low impact sports.
Basically just do what you are happy and able to do, and don't talk yourself out of it.
