I don't think it is out of the question. Weekly mileage is as or more important than what you do on any single day. 20 miles per day is 140 miles per week and I think you can effectively train for a century by riding 100 miles per week. You do need to work in some longer single days leading up to your century. You really should have at least a few days over 50 miles under your belt and a 70 or 80 mile day is a good idea. Also it depends on how much you are willing to suffer.