What distance to run tomorrow?

I’m looking for suggestions on what distance to run tomorrow. A silly sounding question I know, but it is a kind of unique situation.

I’ve wrapped up my first marathon training block that theoretically has me race ready for this weekend, but I’m not doing a race. It sounds crazy to write this out, but I did the training plan to see how my body would react to pushing up my mileage and to get a sense for the challenge of sustaining an 18 week training cycle. I’m planning to keep working at my running and do another training cycle to actually run my first marathon in the fall.

But that leaves me with what to do this weekend. I’ve got my fitness built up and am rested from the taper. I’ve never run to try to set a PR before, just complete different distances for fun. This feels like a great opportunity to pick a distance and see what I can do and learn about my fitness as part of my journey to doing a marathon.

So, what distance would you run if you were in my shoes and wanted to set your first PR? A mile, 5k, 10k, a half, something else?