I think the best answer would be a fartlek by feel. More fun than an endurance run, but less taxing or mentally hard than an interval session. You can run it as hard as you feel on the day and it'd never feel like a chore.
So many here aren't thinking straight. One run per week does not work unless other activity is done in between. The question was asked in such a way to insinuate that nothing else is being done. A runner can't do a workout once per week. They can't run long once per week. Even twice per week would be risky. 3 times is the minimum for reducing injuries and progressing. Monday and Wednesday easy 5 miles and Saturday 8 miles progressive would be okay.
I don't know. I was running 7-12 MPW and my "long" run was a super slow and technical 3-4 mile trail loop once in a while. I'd guess that I could have run a 5k in 29-30 minutes. I really, really wanted that national championship so wasn't doing any distance training at all.