The benifits of exercise are so numberous.
First of all diet won't help you lose weight alone, inorder to get to your goal weight you need to exercise as well (i belive this is acutally a proven fact)
I read an article in a magazine that exercizing can be as effective as perscription drugs in helping depression sufferers due to the hormones that are released.
Exercising helps your body in ways other than weight loss. When you exercise your heart benifits, your less likely to develop osteoperosis, i belive that there are benifits to type 2 diabeties, it can help in cancer prevention.
When I work out I try to read fitness and health magazines to motivate me because reading about all the good it does to my body helps me to get through the work out. and when I'm done I love the way I feel with all the endorphines and hormones raging through my body.
Its good to start out slow though. It appears that you haven't really worked out before (or at least for a long time). Start with things like walking on the treadmill at a slight incline at a comfortable pace. after about a week, bump both up. Or you can do the tread mill, run for as long as you can, but when you can't any more, walk. Keep track of how long you can run and see the increase as time goes by.
Once you've gotten into a grove w/ cardio work out you should definately try to add a weight training program. but i can give you that lecture at another time
good luck and keep us posted!