1. It balances your muscle groups. Focusing on other muscle groups like inner thighs and abs in your cross training lets the muscles you usually use rest while building up others.  You will see improvement in your running/walking as you add more strength to the other muscles.
2. It improves your running/walking aerobic capacity. Without the impact on your body of running or walking, by adding cardio to your cross training you can build on similar benefits of running.
3. It reduces your chance of injury. By balancing your weaker muscles with your stronger ones, you’ll help reduce your chance of injury. Low-impact cross training activities, such as swimming, biking or the like, will lessen the stress on your joints.

Any training program worth its salt will have cross training as a part of it.  Here’s why: We know that running can be hard on the body because of the impact forces that it generates.  You’re hitting that ground at 3 times your body weight.  If you can get benefit without the impact, all the better.

We were just introduced to the Elliptigo, a great cross training option.  A recent study found that riding an ElliptiGo required on average 33% more effort than cycling at the same speed.  It also determined that heart rate responses and perceived exertion levels were similar between the ElliptiGo and running.  You can also do interval training on it just as you would running but without the negative cost of impact.  We liked it so much, we now carry it.