How common is actually holding 300 watts for an hour? I dunno, it's pretty hard to find a place that 300 watts can be done for that length of time with out interuption. The only place I've managed to come close has been on mountain passes & even then...Well, there's still riding to be done that day, 3.64 watts/kg never seems like a good idea.
The times I have done Alp du Zwift however have yielded an actual, measured 288 for a 55 minute run from the starting gate to the top. That sure is awfully close to your question. Close enough for discussion anyway. (Though I'm sure someone would quibble over 12 watts. )
Garmin Insites, comparing myself to men says:
I ride more than 99 out of 100 in duration,
I ride more than 93 out of 100 others in distance,
I ride more than 67 out of 100 in speed,
I ride more than 77 out of 100 in power.
Comparing myself to ALL users in the Garmin Insights data set gives slightly more favorable results.
So theres your answer. Assuming no physiological freakishness or abnormalities on my part: Of male cyclists in the top 1% of time dedicated & top 7% of distance covered it can be done & is likely not uncommon. About 23% do it & it only makes them faster than about 7 in 10 of male cyclists.
Last edited by base2; 02-11-21 at 11:19 AM.