The Plus Athlete has a good article about being last today and I'm going to remember what she said and not kill myself trying to get some unattainable time:
Dead Flat Last doesn't mean you ran any less distance, or trained less, or wanted it less than anyone else. It's not about your time. It's about the fact that you showed up in the first place, trained hard, and completed a distance most people would never even contemplate.
So no matter what, give yourself some credit. Give yourself a chance to finish.