Greetings!
Depending on Jimmy's current athletic state (is he healthy or not?) he may only need one month to prepare for the race.
How he should eat:
Jimmy should eat healthy (fruits + vegetables), and he should make sure to drink lots of water.
He should avoid eating/drinking things that may have a lot of sugar and carbohydrates. This includes things like - unhealthy chips, sodas, treats, etc.
How he should exercise:
Some of the basic exercises he can do is -
*Jogging/running everyday
*A simple sets of stretches
*Push ups, pull ups, sit ups, etc.
Again, this would be an everyday thing, (until the race).
Hope this helps you!
~Lauv