Beautiful loop trail passing several high mountain lakes as well as a back country natural hot springs. Easy access out of the Wilderness Gateway Campground. You will likely run into some folks coming into/out of the hotsprings, but after the first five miles you are almost guaranteed to be solo!

AllTrails link

Awesome loop trail in the Eagle Cap Wilderness area with beautiful high mountain lakes. You will usually encounter a couple backpackers, but after the first eight miles it is mostly solitary running. Great loop for single female trail runners because it isn't overly used, but you do usually encounter a few other people which can be helpful if you are  apprehensive about getting out there solo!

https://www.alltrails.com/trail/us/oregon/wallowa-river-loop-trail?u=i