Hi Skorski,
I recommend following the Masters 40+ Mountain Bike Base Mountain Bike Training Plan for your pre-training plan. This plan sets you up perfectly to follow on with the
Masters 40+ 50 Mile Mountain Bike Personal Record Mountain Bike Training Plan. You will not have time to complete this entire 12-week plan. As you already have good fitness from Fat Birkie I recommend you follow the last few weeks of the plan to fill your time preiod available.
Coach Lynda