Re: What do you do for fitness?
Run, run, run. Running is the main physical strength you need, so it is only natural that running greatly improves your game. You get faster chasing down balls, more overall stamina to play longer and get stronger legs for better footwork.
If you haven't done much physical exercise for a long time, I would take it easy and do some jogging for 10-15 miles a week until you feel comfortable and start finding that length easy. Remember, TAKE IT EASY. It is so easy to get injured or put your body in too much stress especially if you're a smoker (I've never smoked cigarettes, so I personally wouldn't know about this aspect but it would make sense).
After you feel really good being able to run 10-15 miles a week, slowly start increasing your running speed and do a combination of short sprints and longer slower strides since tennis combines both agility and stamina. Then after a while of that, you can up the amount to whatever you feel like.
Federer / Haas / Safin / Gaudio / Kuerten / Youzhny / Nadal / Gonzalez / Ljubicic / Hewitt / Soderling / Wawrinka / Coria / Nalbandian / Kohlschreiber