BMX: Training routine for a beginner?
Off-bike? You might try out plyometric exercises. Plyometrics can help you improve your agility and power. In addition, you'd probably want to work on core strength.

I think that plyometrics would be helpful since BMX races are relatively short and rely a lot on agility and sprinting power.

You also mention that the track is too muddy for riding. So, you could do sprint intervals on pavement. Sprint intervals will improve your stamina and speed. An example would be something like this:


Ride easy for 10 minutes
Sprint for 30 seconds and work your way up to 2 minute sprints
Ride at a comfortable pace for 3 minutes
Repeat the sprint/comfortable pace cycle 4 to 6 times
Finish up by riding at an easy pace for another 10 minutes
Do this 2 or at most 3 times a week but NOT on consecutive days