Rampart and Woodland are the closest trail systems. Both are huge areas of easy to difficult terrain. We hit Rampart frequently and average about 35 to 45 miles a ride, never the same trail twice. Plus there is the Sprucewood bar and grill in the middle of Rampart. Ride in, get a beer, only one though, Rampart is crowded and headons do occur ;) and cheeseburger and ride back.
Then there is the mountain riding. Weekend trips in the summer will be worth the move itself. Crested Butte, Buena Vista, Ouray, Lake City, Telluride, Salida, Steamboat Springs, and more. You can get a day trip in to Breckenridge if you wish (Red Cone and Webster passes, North, Middle and South Forks and more).
You will be only a 5-6 hour drive from Moab Utah, the greatest riding area of all.
There are at least 5 tracks within a 30 to 60 minute drive from the Denver area.
If you want, go to this link of my pictures of CO riding over the past 8 years or so.
http://albums.photopoint.com/j/AlbumList?u=1600327&f=0
You will totally enjoy the riding here.