Q: I'm a runner, but I'm thinking of adding biking to my week. Are these workouts pretty equivalent?
A: Biking is an awesome addition to your running routine and will definitely give your knees a break. Unfortunately, from my understanding and own experience, it does take a bit more time on the bike to get an equivalent aerobic workout. It's totally worth the investment though, and I encourage you to give it a try.
Before you get into this, make sure your bike is correctly fitted. If the fit is wrong, you're going to be uncomfortable and possibly cause more problems with your knee and your back.
I recommend replacing two of your runs per week with a bike ride (assuming you're running about five days per week). If your runs were 45 minutes, plan on 1:15 or so on the bike. This is partly due to the fact that it generally takes longer to get your heart pumping in the aerobic zone on the bike as compared to running.
Be prepared for this to feel very different. The muscles you use for biking are different from your running muscles, and that's the point, but those muscles are likely not as strong and used to being worked repeatedly. Your perceived exertion will feel all out of whack. You may curse your bike. This is okay; be patient with yourself.
As runners, I think we get used to the feeling of always being a little bit sore. Hips protest when we sit too long, knees creak when we stand up, our hamstrings and quads are always asking to be stretched. When you start biking, there is very little soreness, and the soreness you do feel is different. What you will likely notice the most is the fatigue. When I first started biking I was very confused; "I'm so tired! Why am I so tired? I used to be sore and tired, now I am just tired! This is weird!" Weird, but wonderful. That's a sign that you're getting stronger in new ways.
Note: Biking may cause you to consider establishing a relationship with a good chiropractor, especially in the beginning. You may notice some lower back/hip pain after a few weeks/months, depending on how much you are riding. I think this is due to the natural imbalances we have between the right and left side and also from the change in muscles being used to exercise. Keeping up with regular strength training, especially core work, will help minimize problems.