Would it hurt to sign up and just run it as a longer long run for the experience at ~6:40 or so?
I don't think it would hurt, unless you've got a target race too soon afterwards. Here's my example: I raced two marathons within five weeks (2:47, 2:45). The next month I ran a PR 5K. And a week after that, ran another marathon, setting a new PR. During the 37 years since then, I've never bettered either. Very similar age and ability as you -- age 23, the PR times were 16:04 and 2:40:30.