Check out Danny Chew, until he had his fainting episode a couple of years ago he was mostly averaging just under 20K miles a year since the late 70s/early 80s. He was hoping to hit 1 million miles in a lifetime.
Myself personally, I've ridden over 102,000 miles since th start of 2012. I've had 3 years over 20K during that stretch. Doing 20K miles in a year is quite easy. You just have to want to do it. I know of a guy who use to ride that kind of mileage daily going back and forth to work by bicycle each day...like 61, 62, 63 miles round trip each day. It's not that hard. The hardest part about doing something like it is...giving yourself the chance to do it. Once you forget about any potential reason why you couldn't do then you will find it quite easy to do.