A weary racer tends to become sloppy. And when snowshoe racers loses form, they waste energy through excess motion, slow down, and may invite injury. Since strenuous workouts are tiring, carelessness is often inevitable. It's well worth investing in a pair of hockey-style ankle guards for protection. I've seen racers go without this protection and over the course of a long run produce frightful, livid bruises and abrasions on their ankles.
Slight fractures are a distinct possibility too.
You'll notice as you run in snowshoes that you're basically doing what you've always done when you take to the roads for a training run without snowshoes. Mostly, don't become discouraged by your times on snowshoes. They'll be significantly slower. For example, I once snowshoed the first 38 mile leg of the Yukon Jack race many years ago in about 5 hours. I was absolutely wasted after that 38 miler and needed help getting to the hotel.