The best way to improve pace and quickness around the court is probably through doing sprints and footwork drills - some handy drills here: http://tennis.about.com/od/drillspra...orkanimsbs.htm
In terms of improving your stamina I'll tell you what I find to be effective, of course everyone probably has there own preferences, different strokes for different folks.
On top of regular exercise I like to have at least one day a week of a nice long slog (3 hours+). After already playing tennis for an hour+ I do dome sprints and footwork drills, followed by 20 or so (sometimes more) minutes on the gazelle (not the animal
), followed by weights, sit-ups and stretches. I then like to do practice strokes with my 3lb lump-hammer - take care of your wrists if you're doing this. Then a warm down with some stretches and possibly light jogs
I just work off the mentality that if you can make it through a tough 3 hours of exercise that a 3hour+ match shouldn't be too much trouble when you factor in the adrenaline etc that would get pumping in a match.
Hope some of that helps. Also, while some people work without coaches I certainly feel that coaching has been a big help for me (and I reckon that most coaches will do some work on your footwork also).
*I also recently tried one of those workout programs that you see on TV and found it pretty helpful in terms of overall fitness