Back extensions should be added to balance the abdominals. This is a form of training done by world class athletes, typically when they are injured. Surprise, this is not cross training , but is really a form of non load bearing running. It can be used to maintain fitness when impact is not healthy. The running is done in the deep end of the pool using a buoyancy vest.
Many pools have vests which can be rented or a vest can be purchased at the local running stores.
Cycling Cycling is an excellent activity for development of the quadriceps and lateral upper leg muscles. These muscles are often weak in runners and responsible for knee and imbalance muscle injuries in the groin area. Cycling is a great activity on its own allowing you to participate in competitive and touring cycling events as well as duathlons. Make certain that your bike is properly adjusted for your individual height and arm length by a reputable bike shop. Often bike shops or local cycling clubs sponsor group rides for different abilities.