Using my high-tech pencil and paper on the map technique, it looks like the hike from Point of No Return trailhead to the Sand Creek campsite is about 10 miles. They are sandy trails, though, and over half of that was on the trails chewed up by hunters with horses. Nor did I have hiking poles. Such does not make for epic mileage.
I'm not sure how far I got up Sand Creek while exploring. I poked around in a lot of holes, per se. It looks like if one followed the creek all the way into the mountains (past Music Pass to the lakes and springs) it would be about another 15 miles, topping out at about 12000 feet. So, you can make this place into almost anything that you want it to be.
I will also state for the record that though this was a rather short hike it nonetheless pointed out in rather dramatic fashion that I am not yet ready for prime time at this altitude. (Not considering Sand Creek it topped out at just over 9000 feet. I just moved here from Germany, elevation 800 feet. And I felt every one of those 8200 feet of difference, Brother...)
As fate would have it I have driven past the Oregon dunes, but didn't have an opportunity to visit...