You have various options this long before the competition. The classic option is to combine general endurance training - outdoors, indoors, on and off the bike - with strength training. A maximum strength training cycle lasts twelve weeks, so you will be busy from mid-November to mid-February. Then you continue with the Training for your summer destinations.
However, you can also adopt an anti-cyclical approach (inverted periodisation) and insert a VO2max block, for example. This is suitable for maintenance training, but also for improving your aerobic performance if this is a criterion that you want to improve.
A full cycle lasts eight to twelve weeks, with two intensive training sessions per week (simply repeat the 4-week plan above). These units can be perfectly realised indoors. For general endurance training, you can mix in mountain runs, cross-country skiing and other activities with cycling - variety is good for the body!
Functional fitness, strength and mobility should also be part of your routine. You can also work on weaknesses with specialised training such as technique training (cross/MTB riding technique) or fasting training to stimulate fat metabolism.
Zwift 101" is a mini-programme suitable for getting a taste of the world of structured training. "Active Offseason" is a fairly intensive and comprehensive programme for frequent riders with 100 hours in twelve weeks, which also includes basic training. If you go through with it, you are guaranteed to get through the holidays without a break.