Re your comment about average vs cruising speeds, I only note average speeds when I go a set distance with minimal stops, such as commuting. On a tour, I don't look at average.
The way I think of cruise speed, I notice the rough high and low speeds in areas I know are flat or very close. A good stretch for me for that is the bike path between Hamel and Staunton. Flat and straight. Easy effort, 18 mph, moderate effort 20 mph mostly with some 21 or 22.
Or on my way to work, the road is not perfectly flat, but in a long section the grades are minor, like 1/2 % or maybe 1. On the upslope, I tend to go 19' then on the section that goes down at 1/2%, I move up to 23 mph.
Same when I ride around the flight line at base. Theres a very gentle slope up toward the north, a flat at the end and a gentle down going south on the far side. I go 19 up, 20 across the end, and 22-23 down. So I figure my cruise speed is around 20-21, unless I'm tired then it's more like 18.