Sounds great, but I either don’t believe you or I think your judgment is wacky.
1) You claim to have run a 14:10 5K in the early 80s and dipped under 30 minutes for the road 10k. These would have been very respectable national class times for those events and distances so you would have known that you were a step away from being elite. There would have been plenty of decent-sized meets around that you could win with those times and ability. So you would have known that you were an extremely talented runner, regardless of your ‘barely under 30’ as if that was something to be ashamed of.
2) I don’t trust anyone’s assessment of their average speed on a ride who doesn’t talk about terrain or average/normalized power. When you say 18 - 20 mph, usually that means that you were looking down on the flat and that’s typically what you saw. But you’re not reporting the climb where you dropped to 12 mph, etc. The people I know who really ride usually talk about power, kJ, etc. No one’s geeking out over ‘pace’ because it is largely meaningless.
3) I think it’s great you’re fit at 63, but I honestly don’t know anyone who couldn’t just hop on the bike and ride 40 miles in 2 hrs. That’s just a measure of aerobic strength, which you clearly excel at. However, I wonder how long you’d last in a paceline with 45s pulls or any of the other things that cyclists do - 3 x 15 minutes at FTP, 3-4 minute intervals at 400-450W, etc. There are plenty of fit old dudes who come out on rides sporting their knobby calves and varicose veins, but they don’t last long in the 1,2,3 pack, except on Sunday in the group ride.
4) Finally, the last thing you’d have to consider is your ability to handle. As other people have said, you can be the strongest climber in the world, but if you’re not willing to descend at close to 100 km/hr, then you will lose time. It also takes a lot of nerve to set up your final sprint in a crit, most people just don’t have the balls and the knowledge of how to fall when and if you do. In racing, it is inevitable.
So all these tough runner guys who want to say how easy cycling is by comparison have obviously never done any of this, otherwise they’s have some more humility. A 10k bike isn’t gonna help you if you can’t keep the rubber side down.
But hey, if you really want to find out, so find a 40k TT and enter the Eddy division. It’ll answer the question for you, even at age 63. I’d guess about 67 minutes for you.