find a big hill, ride up for 3 minutes, roll down 1, up 3, down 1 and repeat to the top. Turn around and do it again.
This. ^^
I specifically trained last winter for mountain bike racing with the help of a very good trainer who also happened to be an ex pro.
Because of the nature of mountain biking, i.e. explosive power and punching out of corners I was given lots of drills to help with this.
Power revs, can be done on road or mountain bike. At an almost stand still and in a very low gear, accelerate hard for 12 revolutions. Do these every 5 minutes throughout the ride.
Speeders. Throughout the ride sprint as hard as you can for 5 seconds. Again every 5 minutes.
Intervals. 4 minutes hard, 6 minute recovery, repeat.
HIIT, loads of drills about on the internet.
Lots of steady rides too.
These are just a few of what I do.
I’ve gone from being rubish and tail end to finishing well up the order in my class. Last mtb race finished 11th and last week finished 5th in a cx race, 3rd in class.
HTH.