I'm an advocate of spinning, using low gears, to protect the knees. I'm over 200 pounds and I've been riding road and mtb for 30+ years without knee issues.
I'm not fast and lately having trouble getting enough saddle time but 25 miles is just a warm-up. I doubt if you're overtraining in the clinical sense but you may be a bit bored or burned out.
To me, the thing that keeps riding fun is riding with friends or other like-minded people, especially if they are willing to wait after a climb. This is also the best way to get faster, riding with faster riders, as long as they wait if you get dropped.
Don't know if you've tried it but riding bikes in the snow is amazingly fun. I have not tried a fat bike but I did ride a mountain bike in the snow quite a bit and it is a blast. You rides can be shorter and quite intense, depending on the snow (wet or very soft snow isn't great to ride on) and if you have the right clothing you will be fine, even if you have to push some.