For years I have tried to become a runner. I would go for a couple weeks jogging and sprinting everyday but sort of came to resent it. It didn't help that my patience was extremely low. I think that walking is a perfectly fine alternative, that is if you are willing to do interval training. I have found that altering my speed and incline really helps me get a good workout in. The key is cardio, and that doesn't necessarily have to mean running. You could consider cycling. it is a great cardio workout and is extremely easy on the joints.