Pointe Noire de Pormenaz is a great loop to run near Chamonix. I took the train from Cham to Servoz, and then followed the road into town to reach the trailhead. It's a steep route up (lots of power hiking), but the views from the top are spectacular, and you're likely to see far less people than on the trails accessible directly from Chamonix. I looped back down to Servoz via the Refuge de Moede Anterne, but I think it would also be an interesting (and longer) route to drop down, and then climb back up over the Col du Brevant back to Chamonix. The loop ended up being around 14 miles with 5400 ft of elevation gain.
