If you’d like another data point, I’m 32, have been cycling most of my life but only ‘seriously’ for the last couple of years, ride a hybrid with no panniers, and I average just under 15mph on my (very hilly) commute. That’s a moving average. I couldn’t carry on a long conversation on the steepest hills, otherwise I’d say I maintain a comfortable (for me) pace. On the mile or so that is near totally flat, I can maintain 20mph comfortably (barring severe headwind), not sure for how many miles I could keep that up though. I will say though that if I meet a really serious rider, I will struggle to keep up, even on their wheel.
I’d say for you it’s more about being able to cope with the distance and elevation than the speed you go at. I recently did my first 50-mile ride at roughly the above speeds, and I was pretty sore (and slow) by the end of it.
Remember, there will always be someone faster than you out there.
I friend I do a lot of cycling with (both regular fitness/recreation and touring) rides much more and faster than me, and I know he slows down when he rides with me so we can carry on a conversation. I would say when we are together for a fitness ride we are in the neighborhood of 15-17 mph average on flat terrain uninterrupted. But he regularly stealth brags about maintaining 20-23 when riding alone, and reports trying to keep up with an elite group riding around 25 (but getting dropped). He is also around 25 lbs lighter than me.
I don’t worry much about getting passed, or how fast I am going, especially on a tour - the purpose of riding (like your Utah trip) is not to go thru it fast, but to experience the road at a slower pace than in a car. It is more about the journey, not the destination. Slow is relative.
This was a good question as I was curious about my riding speed. I’m not too serious about riding as is evidenced by my nearly 30 year old aluminum bike with toe clips and old style (friction?) shifters. I just try to build up a beer deficit that I can take care of on the weekend. Three or four days a week (gym 2 days) I do a 16 mile loop at 17mph. Its flat except for four bridges with about 60’-70’ elevation change bottom to top on each. My thighs are usually screaming by the top, as I push the hills (or what pass for hill in south Jersey). At 60 years old I guess I’m in the ballpark. It feels like a decent workout - faster than conversational but not gasping either. BTW, the wind sucks.