MTB races are always a PITA to warm up at. You often end up getting what you can and hoping for the best. You can never be sure when the race will begin. Your car is parked no where near the race course. Between registration, restrooms, and other prep, it is a real challenge.
I thought the scenario at the chainstretcher worked well. The road to the start was uphill which provided the resistance to get you HR up. The course went immediately uphill from the beginning. I went back and fourth on the road a few times. Then, I hit the course and rode the first climb twice. Since I did not pre-ride, this provided warm up and allowed me to scout the lines up the fire road.
No wonder going remotely hard from the gun feels so painful.
I know a great interval workout to help improve your MTB starts. It is from Joe Friel's Training Bible.
After adequate warm up, ride at maximum effort for one minute followed by 6-10 minutes of tempo pace or above. Repeat until dead. As your fitness improves, increase the intensity of the start and/or the length of the tempo segment.
Tempo is a sub-maximum effort but still quite high.
This is how every MTB race starts. Guys gun it for position and then settle in and see how long they can hold the pace. The rider that can hold it the longest wins.
okay so you do your warmup -- how long do you allow between the end of the warmup and the start of the race?
As little time as possible. Do your warmup near the start so that you can periodically check to see if the promoter is getting ready to call riders up to the line.