Discussion: It would probably be more f...

It would probably be more fair to yourself in considering how you would have done in the race compared to the other old guys to note that had you been riding it in a race instead of solo, you would almost certainly have ridden it quite a bit faster. It is an almost irresistible bit of human nature to push harder when there are other people around (one interesting consequence of this is that orienteers are a lot more likely to run faster than they can think when there are a bunch of other people in the woods right nearby). It is also worth noting that most of the folks participating in a mountain bike race have mountain biking as a primary activity, not a secondary one. Bicycling (like golf, or orienteering) depends a lot more on technique than running. You have been doing a lot more of it in the last year or two than previously--it is an area where you might well see your performance actually improve for a few years because of technique improving faster than muscles are degrading.

I was thinking that under race conditions, other people around, that ought to be good for about 5 minutes. And last Saturday was a nice cool morning. But still would be pretty slow.

Note that this race (well, technically a time trial) is all on pavement. Obviously there are skills involved in riding efficiently, other than just being able to crank out a lot of watts, but I don't think near as much so as in mountain biking. I'd be a lot further behind in that.