There are lots of trails here for almost all skill levels (although I don't recommend it for first timers). You can either ride up the mountain or take the lift to the top (costs about $25 for a day pass to ride the lift). A map can be found at: http://www.keystone.snow.com/images/map.mtnbike.jpg. The green trails are long and rooty, the blues are steep and also rooty / rocky, and the blacks are dominated by guys on $7000 downhill bikes. A single run down the mountain takes anywhere from 30 to 60 minutes. Its a fun ride no matter which trails you take.
From the Keystone Website: From Denver, take Interstate 70 west through the Eisenhower Tunnel to Dillon, exit 205. Continue six miles east on U.S. Highway 6.
http://www.singletracks.com/item.php?c=1&i=58