The best single overall tennis training site is Sports Fitnness Advisor Tennis Fitness Advisor
http://www.sport-fitness-advisor.com/tennis-training.html
The best advice on running to get fit is the following:
"When training the players the USTA works with, we usually do some sort of "running" four to five times a week. The running session usually lasts between 20 – 40 minutes, but there is a lot of variety in the types of running we do.
You’ll note that we put running in quotation marks, because much of what we do is different from the long, slow distance running many tennis players are familiar with – there is some long distance running, but the “running” sessions also involve footwork/tennis agility work, or interval runs. The type of running depends upon the periodized strength and conditioning schedule of the player.
Generally, the long distance running and longer interval repeats (400s and 800s) are done during the preparation phase when you are getting ready for the season. Shorter, higher intensity intervals (20s, 40s, 60s, 100s, 200s, and 400s) and on-court footwork/tennis agility are the main focus during the pre-competition phase in the weeks leading up to main competition or competitions. During the competition phase of the season, on-court footwork/tennis agility is the “running” focus.
Recognizing that each player is an individual, we adjust the plan depending upon the player’s cardiovascular endurance, agility and their physical and physiological strengths and weaknesses."
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http://www.usta.com/Improve-Your-Game/Health-Fitness/Training-and-Exercise/Conditioning/
High Intensity Interval Training (HIIT):
http://www.intervaltraining.net/hiit.html
AgilityDrills:
http://assets.usta.com/assets/1/USTA_Import/USTA/dps/doc_437_269.pdf
Best set of arm exercises to help avoid shoulder, elbow and wrist injury from too much tennis:
Thrower's Ten Exercise Drills:
http://www.eteamz.com/palyvolleyball/files/throwerstenexercises.pdf
Recognize that off court training can improve your fitness, strength, speed and reaction time.
But only coming out of a split step every time your opponent hits the ball and spending lots of time on court at the baseline, practicing approach shots and volleys on the run, and working on your returns and serve will translate that increased fitness into usable "muscle memory" for your tennis game.
So if you don't have an opponent, a good session running yourself by hitting against the wall can also prepare your body balance and reaction as well as your strokes.
