If I had to pick one thing I to work on off the bike to be faster at cycling (I’d like to pick about 4 things) it would have to be core work. So many cyclists are incredibly weak in their core and it affects so much of their riding. The core muscles have so much connection to your legs and other support areas that it is often that the back gives out way before the legs do on a tough climb, cross course, or mountain bike ride. Therefore, your legs and lungs don’t become the limiter, rather your back and abs become the reason you have to slow down. Additionally, cyclist specific core work usually involves excersizes that end up lengthening your hamstrings (always a good thing for cyclists).
The cool thing is that it doesn’t take much to develop a stronger core. 15 minutes x 3 times a week of very specific work can pay major dividends. There is no better time to get into your core than the fall. Preparation now could set you up for a good spring/summer season.