One of the most important parts of a cross race is the 1st minute. You want to get to that 1st set of barriers or run-up in the top 10 or you will be stuck behind a bunch of people as the leaders will never be seen again.

From a complete stop. Sprint out of the saddle for 15 seconds, then sit for 30 seconds going all out. Rest for 75 seconds. Do 5-10 of these. I find that over 10 is diminishing returns for me. My HR goes from 111 to 185 during these intervals. I do them on my road bike on the road, because if I do them on the road with my cross bike I run out of gears with a 42×12. Ultimately it would be the best to do these on a cross course and with your foot out of the pedals, just like you are starting a race.