Fitness question

I'm often that guy in front of you, I am heavy for a cyclist and get hot and fatigued on long climbs as a result, but am fast downhill and reasonable on the flat.

I'm very strong, but not all that fit, I push big gears at a low cadence.

I think you just need more training into your legs, my CV fitness is crap these days, so unless it's disaster cramp, my legs rarely get sore as my heart and lungs don't allow them to extend theirselves!

Mix your routes with longer flatter stages, set a higher average, I reckon you will start to find a different muscle burn.

It sounds like he was riding to power, or at least to a measured effort. It's the most intelligent way to get a reasonably fast time.

The 'mistake' many people make is to dig in for big efforts up the climbs, or even short rises. It's very easy to suddenly be doing 500 or 600 watts and after 20-30 seconds you start to feel it. Keep doing that over the course of several hours and your legs will know it.
The problem is compounded when they get to the top and then ease off and coast down. So they lose the seconds they might have gained, but have put a whole load of lactate in their muscles.

On the other hand if you back off on the inclines and keep it steady but very importantly you also maintain that effort downhill and on the flat you'll be able to keep it going for longer and maintain a higher average speed.

I found using weights while doing squats helped me with climbing (although not at the crazy type.of climbing that you managed).

At what stage of the event did your mind start to waver?