The most I have done is 21,347 according to
RWGPS. GPS on the actual ride was closer to 26.5k, so close, but not enough climbing to qualify. I finished that one in 37h25m, and it was my hardest ride ever with constant up-and-down short, steep hills. Seems like something with big long steady climbs would be the way to do it, and the 60 hour time limit is very generous.