Here in the greater Denver area, I freely admit to being spoiled: thanks to the local MUP network, it is possible to put together not one two road-free metric century rides, but several with only a few miles of repetition. Branch onto the roads and there's that much more pavement available. Finally, both MUP and paved roads lead to gravel. Or grab your MTB and head to the CO Trail, which starts nearby and runs some ~500mi to Durango on the border with NM.
Bored with local options? Hop the Bustang with your bike or drive it a couple hours west, to Frisco. From there, MUPs lead around the adjacent Dillon reservoir and to several adjacent ski resorts. In the summer, these are largely bike resorts, with MTB and DH options galore, depending on where you look. You can also opt to take a MUP to the top of Vail pass if your legs and lungs are up for it: the ascent is 1600ft/488m over 25mi/40km, which sounds okay until you consider that you start climbing at 9,000ft/2700m altitude.
Turn back after enjoying the views from the top of the pass. The descent is delicious. There's no other word for it. Alternatively, you can ride on through to the town of Vail. Head further into the mountains and the options proliferate.
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