The Colorado Trail
Most of the 500 mile Colorado Trail, which goes from Denver to Durango, is not accessible. However, there are a few places where you can follow it for short distances, at least on foot. Here is their website.
The start of the Colorado Trail is at Waterton Canyon, a wide dirt road beside the South Platte River. Here is another link, and still another. I believe the first 6 miles are wheelchair accessible and there is an accessible fishing pier about a mile and a half from the parking lot. This is a road, not a trail, and you will meet a lot of bicyclists and runners, occasional cars and rattlesnakes. It can be very hot in the sun.
We walked about 1/2 mile east on the Colorado Trail from Kenosha Pass. We could have gone farther as it was easy walking, but the way was blocked by two fallen trees. This is a convenient place to enjoy the aspen colors in the fall. Parking lot: N39° 24.814′ W105° 45.277′ The trail going west from the highway becomes difficult almost immediately.
Another place to access the Colorado Trail is from the summit of Marshall Pass. It is easy walking for at least 1/2 mile going North from the summit.