I just did a 1 hour walk, 1 hour is reasonable, the reason why I dont commit to something meager like 20 minutes is that its too little to make a meaningful difference, I experimented a while back with walking everyday and was walking for 4 hours everyday, and though I had amazing results it wasn't sustainable, I dont want to walk for 4 hours everyday for the rest of my life, I figure if I can get in the habit of walking 2 hours a day it should be good.