join a gym or exercise class
Join a gym and attend some of the classes regularily. Or find a class to sign up for through the city or rec center. Then it can become more of a social thing and you can make some new friends. Then it is easier to exercise because you want to see friends.
I am a morning person and would suggest that you try to find a morning class and then work on your dissertation afterward. You will probably think better and have a feeling of accomplishment afterward. If you are not a morning person, find an afternoon or evening class that will give you a break.
Try to make exercise a habit and just do it, like brushing your teeth. You might not like it at first, but just keep doing it because you should. Know that you will have good days and bad days.
Another thing I do is download entertaining books to my iPod. Then only listen to them when I exercise. If the book is good, I want to listen to it more and it makes exercise more fun.