I am going to go against the grain and say a rode bike even a cheap one will do much faster than a mountain bike. A cheap road bike with 700-23 tires at 110 psi on hard blacktop you should be able to ride the 10 miles in 30 minutes. I am a runner first and while I have great edurance I am not a fast runner. I do ride serious but on my son's cheap Sekia 1000 I could easily go 20mph for 30 minutes if not faster. I am not sure what kind of shape your in but if you can run miles at 8-9 minute pace training you can ride road bike at 20 mph for 30 minutes. A mountain bike is like running in your wing tip dress shoes they work but are not nearly as smooth and fast. Borrow a bike and try it. 12 miles you should be able to do easily in less than 50 minutes.