When running, try to land on your heel and push off your toe to minimize strain on your knees. Get good shoes. Shoes with poor support can impair your training, cause you to tire easily, or strain or damage leg muscles. Because feet tend to swell when you workout or run, go shoe shopping at the end of the day, so that you get the most comfortable fit. Be sure to include anaerobic exercise in your training program.
Weight training builds muscle.
For instance, strong abdominal muscles help runners by decreasing strain on the back and knees. Find your limits and build from there. Starting with a 20 km jog on your first day when your body can only handle 2 km can be both damaging and discouraging. Make sure that you warm up and cool down properly with the appropriate stretches. Otherwise you can easily pull a muscle.