A smooth, low impact walk is a great way to stay fit. Chi Walking is a low impact style that can keep your fitness level up. Believe me, walking a 10 minute mile requires a high fitness level and to maintain that effort for a half marathon distance is equivalent to a 22+mph average for 2 hours on the bike.

I had 8 weeks of no biking, low impact walking exercise while wearing a neck collar for a cracked C-6 then had my neurosurgeon tell me the crack was healed but now I had compression/slippage issues with C-5/C-6/C-7 and another 8 weeks of the collar 24/7. Again the only thing I could do was walk for fear of paralysis. If you're serious about staying fit and you can walk, this is a great way to do so.