I would worry more about physical weight than about how in shape you are. As a practical matter, the majority of actual (not symptomatic of wanting to go home) debilitating injuries are to the knees. How much you weigh is directly correlative of how much damage you knees take on downhills. Being in shape has nothing to do with that damage.
The trail, for all intents and purposes, is the only place to get fit for the trail. It is what GA is for. Teaching your body and mind how to hike a long hike. But you can't do that if your knees are killing you.
Just think of the downhills when you think of binging. Good luck.