Hm... All over the map, really. Depends on the ride and the bike. I don't really go on "training" rides any more, I just get on the bike and ride. Sure, I still like to go fast, but I don't do any structured training. I enjoy it just fine.
My only MTB is a rigid single speed monster cross that I mostly just ride on the gravel service roads and single-track trails that I'm cutting into the woods at my work site. If I stay off the single track, average is around 12-13. If I get into the twisties, it drops to around 9. These rides are mostly an hour-and-a-half.
Road rides on the roadie, about 19-20. If I'm on the gravel bike with 28mm road tires, it drops to around 18 or slightly less. Gravel bike on gravel roads (which around here usually means extended climbs, sometimes VERY steep necessitating that 34/32 gear combo) drop down to about 12.
I think the last time I rode aero was the last tri I did, 2 years ago. In fact the tri bike frame I finished last year still doesn't even have a chain on it. When I did hit the tri course those last couple years, though (still no structured training), I was around 22-23 on the course. My best ever 40K TT (on a road bike with clip-ons and Tri-Spokes) was 54:35 (27.3mph). That was a long time ago. And yeah, it hurt.