If you really want to train you can do intervals, practice dismounts, running with the bike, try different tyre pressures, practice accelerating out of corners…
Or you could ride each race as hard as you can and see what happens, get the heart rate up, try and do better than last week.
For me if I didn’t race cross in the winter I wouldn’t be riding my mtb I’d be on the sofa so I look at it as free riding. Take pleasure in watching the people who are properly good at it be good at it and rock around giving them some pack fill to beat.