I hear you about not being 20 anymore. Weight is the big thing but it really does matter what you eat. Lots of meat slows you down - literaly! Too much fat clogs the small capilaries in your brain and that's also why you get tired so fast. I am 52 years old and can still run up a 20 foot ladder and not be winded. Eat mediteranian with lots of whole grains, fruits, vegetables, and you will outwork your apprentices. Meat makes arthritis worse too and that can be scary.
findthelight is quite right. Good diet and easy exercise is good for you.
Best thing I did was get a dog. I got a retierd working dog from work. I has been used to being active so we walk it a minimum 2 miles a day. 5 on a sat sun. If i get lazy and dont take him out at the usual times ..... well lets just say I get a talking to from him :-)