There is a bike shop in Old Town Alexandria called Wheel Nuts, and they rent bikes.

http://wheelnutsbikeshop.com/1/?page_id=2

I haven't actually met the owner, but he is a member of my bike club. I expect you could call them for up to date information about trail conditions.

Also for general info on rides in the DC area, including the Mount Vernon trail, there's Bike Washington. www.bikewashington.org.