How do you get ready for the pace of a motocross race

If you mean being in amoungst all the bikes and the speed. Practise one on one with a rider who is better than you and get them to push you and if possible do some practice starts with other riders who may be attending practice days. It is difficult as until you experience the bashing of bars and push and shove of the first corner it is not something that is easy to prepare for.

ride with a bunch of your friends before a race or watch the race before yours so you know the pace of the track

http://atomsfamily.net/ has some good riding tips and race prep tips