More than one discipline?

I think if your fit enough to be at the sharp end of a 3rd Cat race and also knock out a 24-25 minute 10TT, then you’ll be shifting at an endurance MTB event. Not podium level but you’ll be way nearer the top of the results than the bottom.
TBH if you are serious about racing I wouldn’t do much mtb at all. Even the top mtb riders don’t ride mtb more than once a week typically.

Kryton, to save you £6, you’ve broadly got rule #70 already when you said

its a measure of personal achievement

But you just have to find some part of it fun. There is no other point in doing it. Something has to get the adenaline going. The training might get you nearer to what you find fun. If you need to win to get fun out of it, then you might need to adjust your expectations.

For me, I turned up at my first open road race a couple of weekends ago. Sat in the bunch, concentrating hard on the wheels around, the sound of the bunch like a swarm of bees…..absolutely fantastic!

buy a fat bike,