The trail to Willis Lake in the San Isabel National Forest, is a very lightly traveled trail, so much so that on parts of the trail there are fresh new grasses growing in the middle of trail. It's a very nice trail, steep in places, but very well taken care of. This trail came in at 12 miles for me. You do get to spend a little time on the Colorado Trail and the Continental Divide Trail, so that's kind of cool.
It was mid-September when I went, so Fall is in the air. The air is brisk, there is a constant, noticeable wind, the Aspens are changing and the brush is rusting giving the valley floor dramatic contrasting textures. I got lucky with the weather, it was forecast to be cloudy all day, but with the wind it there were just a few clouds racing by through a big blue sky. It is a really pretty trail; I'm surprised I only ran into one person on the entire hike. It was a very peaceful hike with some incredible views. Checkout my GPS tracks here.