Average speed is extremely dependant on:
- Your fitness (main factor)
- Weather (particularly wind)
- Road surface quality
- Interruptions like traffic lights, dog-walkers on bike-lanes
- Accumulated fatigue over multiple days
- How hilly the terrain is (although this can be balanced out by the faster descent)
As you mentioned, best way to see is using a GPS and seeing how fast you go.. I've found over the course of about 6-months of riding, my average speed over long rides is around the average of my shorter rides (I'm classifying "long" as around 150-200km, and "short" as maybe 30-80km)
For example, here is a plot of my distances vs average speed:
(the axis's are in km/h and km)
The >50km rides averaging 25-30km/h are mostly group rides. Ignoring those, beyond about 80km begin to converge to an average of 20km/h (although at 80km I've ranged from about 15-25km/h, but this includes when I just started riding..)
These numbers are all specific to me, and even still they vary (particularly over time):
These averages are spread over a few different bikes (start to April was on a hybrid bike, April to mid May was on one road bike, and the rest was on a different road bike) - but, the spikes are almost all related to either terrain (there's a large dip in July related to a Strava hill-climbing challenge), fatigue (the dip in August was another Strava challenge, to cycle long distances over consecutive days), or other factors mentioned above
Sorry for the rather rambly answer, but it hopefully conveys that average speed depends on a lot of factors, and it's hard to give a specific answer