Here's a thing to try? Hare Scrambles.

[xvxax]

That's really true. When I am being chased hard or on the first lap, I have to fight really hard not to tense up. If I get tense, I get exhausted within 20 minutes or less. The key to lasting is honestly being as smooth as possible and just maximizing speed with minimizing body movement and staying relaxed and loose. If you thrash like it's a motocross and slam the brakes on every turn and power out like crazy you're going to get tired. 2nd most important thing to remember is probably looking ahead and analyzing the course because courses in the mid-atlantic are always muddy and slipping around is fatiguing too.

Ride some more trails and get the hang of it first.