Hey Mike....I usually fatigue at 15-20 minutes, then change to the recumbent bike for 15-20. If I keep my legs moving I can usually get thru the regimen. Where my biggest problem lies is when I have to stand still ( waiting for a clerk or in line somewhere) after about 10-15 mins my legs start to burn and the muscles get so tight that it is difficult to walk. If I can't sit down after 15 min I am in trouble, sitting for extended periods makes my legs get stiff and when I stand up I have to be careful until I can walk it off.