Ease yourself into the training, don't kill yourself off the bat, it wont be good because you'll be sore more often than you have time to train. But I would recommend starting off with about a mile of running every day, increase it by 1 mile whenever you feel ready. Hit the gym, high reps and sets, lower weight, don't worry about what other people think about you. Most importantly... get some seat time on the bike. My favorite drill that I made for myself, take a lap as fast as you can, then ride the bike to a top of a hill, run down the hill and back up while still in your gear, do 10 push-ups, 10 up-downs, and 10 crunches, and then put away another lap as quick as you feel comfortable. Fitness is the biggest part of racing motocross, so many people misconceive that "the bike does all the work" and I and anyone else who rides/races knows that is far from the truth. Make sure you're having fun too. Motocross is definitely not for everyone, but those who find joy in it, absolutely love it.
11 years of racing experience.