That sounds like a really cool event. Tough too!! I recommend following these two plans after your Base plan to achieve your 100 miler goal in September.
Masters 40+ MTB Cross Country Build Peak and Race Training Plan
Masters 40+ MTB 100 Mile Personal Record Training Plan
Adding a speed building block with the Build, Peak and Race plan between your Base training and 100 miler training will be of great benefit to you.
Lynda