I just came back from 7 weeks of trekking in Patagonia and so many times I wished that I had brought my bike with me. I would recommend riding the backside of the Torres Del Paine from Refugio Las Torres to Refugio Dickson and back. If you stay in the refugios you don't need to carry food and camping equipment. The section from Refugio Las Torres to Campamento Chileno would also be a sick ride. The trails around El Chalten would also make for unbelievable riding. Particularly the descent from Campamento Poincenot to El Chalten town. Unbelievable singletrack! The only issue is that these are all busy hiking trails so you would need to get up early to ensure you don't run anyone down!