When discussing average speed, it would be nice to mention whether it's moving time or elapsed. My best time for an Imperial double was a hair under 12 hrs., moving time a hair over 10 hrs. That was in my late 50s. I had a tailwind for some of it and did a lot of drafting. On a hilly 400k, 18.5 hrs. elapsed, on a 400k with only 6600', a hair under 15 hours. On a 200k, over 6000', a hair under 8 hours. Brevet times are always elapsed. These were all in my early 60s. On a 154 mile, 9400' event ride which I've done many times, ~16mph saddle time in my late 50s, down to 14.9 now, so 9-10.25 hrs. moving, from mid 50s to mid 70s. Elapsed times about an hour more than that, so down to a little less than 14 mph now. This is such a hilly ride that there's very little drafting, the riders being so spread out.
These days, local solo rides on my single, usually ~50'/mile, ~50 mile rides,14-16mph depending on mood and conditioning.
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Results matter