With standard gearing on a stock mountain bike, you wouldn't be able to pedal fast enough to go much more than 45 or 50 mph (and even then, your cadence would have to be well over 150), so most of your speed will come from coasting. And the steepness of your hill will determine your speed. One mile really isn't enough to gain enough momemtum to get going very fast. You might be able to hit 50, but that all depends on the gradient of the hill.
There's a hill near me that's about 3 miles long and about an 8-9% grade. I've managed to hit 50 a few times coming down. The auto road for Whiteface Mountain is about 8 miles and an average 8% grade. They open it up to bikes a couple times a year. The upper half is a bit too rough to get up to full speed, but on the lower half I've hit 60.
