Most people have a preconceived notion about how running can tear up your knees. Most people are right when they worry about that because most people don't know how to run properly. It's important to understand how to take care of yourself by running properly and taking care of yourself through technique.
There's also another way that you can take care of yourself without slowing down the pace and working on form. That is to ride a bicycle. In riding a bicycle, because of the way the bicycle is designed, you strain the muscle, but you don't put impact on your knee. The thing is, because of the circular motion and the wheels going round and round, there isn't a jerk as the energy is a continuous motion. Now in running, we are not perfect and when we go slow, the thing is that most of us have that slight impact as we run, which is why it's important to protect your feet and run properly. Now the thing about protecting yourself is that giving yourself a cushion changes your form slightly, so it's very important to understand that and work hard to make it so that you aren't putting too much pressure on the joints.
I rode my bicycle today, and it was great. I was listening to music and was on the greenway, pushing not too fast, but enough to raise my heart rate and make it somewhat of a workout. I rode 7.55 miles in 34:29, an average of 13.1 miles per hour (which is the pace that Ryan Hall ran his American record smashing half marathon). Funny that my medium bike is his running speed.
When I ride my bike, the thing that happens is that the strain is on the muscles, forcing them to become stronger without damaging the joints themselves. It's important to strengthen them because after the race, I was pretty much useless and my knees weren't able to take running because of the little pounding that I would have to do. In cycling I trained my legs hard so that I would be able to support myself in longer runs. Eventually, I want to get to a point where running a 50 mile race is a walk in the park, but that'll be up to my training. I'll have to make sure that I work on all the support muscles in order to support myself as I run this race. It's not just about the science of what you put in your body nutritionally, but it's also about properly training your body and conditioning them to support each other towards your goal.