Trail Running After Knee Injury?

Three weeks ago I was on my dirt bike and fell over in some deep sticky mud. Class 3 tears on MCL and PCL, Class 1 and 2 tears on MPFL and meniscus. It's been three weeks and I can bear weight on it just fine, but my range of motion has not improved one bit since over two weeks ago. I can't straighten my leg much past 10° and I can't bend it past 90° even if I try to force it. Surgeon said it will all heal on its own. Physical therapist says my PCL is non-functional and likely never will be without surgery.

I'm not much of a runner, did a couple 50k trail runs a few months after discovering my hiking pace is faster than at least 30% of the finishers of hilly ultras, really loved it and want to get into more, would love to be able to do some 100 mile races and not be limited by my body or injuries. My main sport is long distance bicycle riding and 24+ hour events on the bike, and I'm fairly confident I will eventually be able to get back on my bike and ride OK. However, I'm not convinced that this knee is going to just magically get better one day and let me start running.

I already have terrible awful horrible issues with shin splints, it has always been the one and only limiting factor in my running training, so I know that is going to be hell to deal with if I have to also fight with this knee.

What has your experience been with MCL and PCL injury recovery and performance once recovered? I don't want to reinjure it while boulder hopping down a steep trail dropping over 1000' of elevation per mile, or blow it out on a 6 mile downhill dash. I'm 34 and in relatively good fitness and have never had a knee injury before. Should I get worried?