I had an accident where I broke my collarbone. I was totally out of cycling for 13 weeks. Before the accident I was racing in local MTB races. I'm still an amateur but I had good endurance to be a medium level racer (last cardiovascular tests score good / very good in general).
How do I get back to business? Should I start my training with easy, longer rides (base), or should I continue my training with a higher load?
Since you don't have a big Winter racing programme lined up, you can afford (you have time) to take a low risk approach.
So I recommend a gentle test ride of 10 to 20 kms to start with. Avoid jumps, mannies or any other tricks on this ride. Decide what distance you think is good for you, and keep to that decision. Don't go further because you feel good. See how you feel at the end of the ride, and the next day.
Take a day off riding to plan your get fit programme, and recover from the first ride. Use what you've learned from the first ride to guide you.
Plan to break your training up, partly for variety, but partly to keep each session focused. I'm thinking you'll need rides /sessions such as
Aerobic / endurance training. That could be SLK's - Long Slow Kms.
Strength / hill training (think intervals, or hill repeats).
Skills training. Partly for fun. But how did you break your collar bone? Is there a skill you could practice that would avoid that outcome?
Plan to train every second day, until your are feeling strong again. Remember to include recovery rides when the training is leaving you feeling stiff the next day. And / or get muscular massage.
There are many other answers that can help you under the training tag.
Good luck.
