As above, I rely a lot on my HRM for longer rides. No long mtb rides recently, ~160ish miles ridden over the weekend though, 105 on the road on Saturday. Felt fresh enough to go to Cannock Chase on the Sunday and still get some personal bests.
Spiking your heart rate up to crazy levels and burning off sugars, is the sort of thing you can only do a few times per ride, it’ll wear you down quick. Diet before and after big rides is one you need to be careful with as well, fuel up! Protein afterwards if you need to be riding again soon.
Something with electrolytes in while riding. If you’re not used to sugary energy drinks, don’t hammer them during the event.