After consulting veloviewer here's some numbers for me:

Median average speed: 24.1kmh (14.96mph)
Best average speed (not in a race/group): 28.8kmh (17.89mph) over 1h47m

It's interesting to look at Strava's estimated power curve for that ride. Note how Strava thinks I can hold 250W for only 4 minutes but I can do 180W for an hour and three quarters. By that graph the difference between the power I can hold for say 30 minutes verses an 1 hour and 3/4 is only about 25 watts.



Which is typical really. For most sports a 1-2 hour effort and a 6-8 hour effort would both be considered endurance efforts. It's only when you get into very short time frames (say 5-10 minutes) that you can start to get significant boosts in your power output and thus your speed.

Here is an interesting comparison.

The world record for the 5km running race is 23.77kmh ( for 12m 37 sec) while the 42.21km marathon is 20.59kmh ( for 2h 3min). The 5km run is only one eighth the distance but the average speed is only 15% better. The 800m is run at 28.5kmh (over 1 min 41 sec) which is a significant increase on the 5km pace but it is only less than 2 minutes that the runners can hold that power and pace.