If I were to ride solo on a pancake flat, smooth road with no wind, I usually can maintain about 23 mph comfortably as long as the flat lasts, but in Oregon, that's usually not that far, maybe 10 miles or so. Over 30 miles riding solo over rolling terrain, I'm doing pretty well if I average 20-21 mph. I've had the occasional ride where I've averaged 24-25 over 20 km, and the fastest I've ever ridden was with two other guys for 30 miles where we averaged just under 28 mph.

I would say the range most cyclists that have ridden seriously for at least two years fall into is 16-24 mph, riding solo over a longer-than-TT distance, but that's just a guess.

All these numbers really mean very little about one's fitness or anything. There are too many other variables.