About an hour east of Denver. There's also a relatively big kart\sumo track about 30m north of Denver called IMI which is fun for smaller bikes and it's only $40, show up anytime.
There are also two to the south which I haven't been to, Pikes Peak International Raceway (PPIR), which I've heard less than great things about, and Pueblo Motorsports Park (PMP), which is supposed to be pretty good.