Bring plenty of water as access is limited to refill at East Lake campground about half way through the run. When I ran it in a light snowpack year, there were still patches of snow for a half mile on the northern edge of the rim.
The Crater Rim Trail #3957 circumnavigates the Newberry Volcano Rim in Central Oregon. The high desert trail features sandy singletrack with views of the Cascades in the distance and both Paulina and East Lakes down below.
There are trail maps available on site and on the internet, but the established trails are all very well marked and linking them together is pretty clear.
The eastern edge of this route allows for an option to stay up high along the rim, or to drop down close to the lake if you need to refill with water near the campground.
You can take a few side treks to reach both Paulina Peak
and North Paulina Peak
to extend the overall distance to around 21 miles, or use the outermost trails to extend your run to 23-24+ miles.