If you want to use resistance bands to work the lower body you need to realize that your thighs are the biggest muscles in your body. You want to make sure that any resistance bands you buy are strong enough so that they don't break during exercise.
Most resistance bands you see in gyms are very low quality and are not strong enough to withstand repeated stress. Bodylastics is the clear leader in this market, and sells a very durable and versatile resistance bands set.
Since I started training for a marathon, people keep warning me that I will ruin my knees or my feet. The reality is that knee and feet injuries are prevalent. I came across a very interesting study in the New York Times recently, that provided some insight into this question. A group of Austrian researchers had studied the knees of marathon runners over a 10 year period. The following is a summary of the study and the findings.
"OBJECTIVE: The aim of the study was to evaluate long-term damage in the internal structures of the knee joints of recreational long-distance runners.
MATERIALS AND METHODS: Ten years after their participation in a baseline study concerning their knee joints, seven long-distance runners and one who had given up long-distance running were invited to participate in a repeat magnetic resonance imaging (MRI) investigation. The same evaluation criteria and the same technical equipment were used, and the results of the two investigations were compared.
RESULT: No adverse long-term consequences were observed in six of the seven active runners, regardless of pre-existing damage at the baseline investigation. In one case the arthrotic changes were progressive in nature. The person who had given up running presented with severe deterioration of the internal structures of the knee joint.
CONCLUSION: Non-physiological maximal loads secondary to the marathon race do not cause any permanent damage in the internal structures of the knee joint in individuals without significant pre-existing damage. A disposition for premature arthrosis was not registered in the population investigated. A protective value of long distance running on the internal structures of the knee joint is discussed."
Other studies have shown basically the same thing- that running does not cause your knees to wear out, and that running often helps improve the long term health profile of the knee. The important caveat here is that if you are overweight or running in an improper form, you can develop serious injuries.