The longest "flat" ride I can think of in Colorado is the 40 to 50 miles from Aspen to Glenwood Springs. It's right in the middle of the mountains but it only drops 462 feet over 41 miles (11feet/mile). Of course if you get off the trail to go anywhere else, things go south quickly.
Denver has the Platte River Trail which drops 700 feet in 37 miles (19 feet/mile) and the Cherry Creek Trail which is 700 feet in 27 miles (26 feet/mile). Again, any rides away from those trails get steep fast.
For true dead flatness, the Highline Canal Trail is a good bet. 50 miles, 530 feet or about 10 feet per mile.