I think you need to up your average distance but as slowly as you can. Any running is better than none (as long as you aren't injured) but those very short runs are probably not going to stretch you enough to help at this point.
You need your body to keep going when you are tired. When you hit mile 20 on the day your body will at some point start screaming for you to stop (if it hasn't already) and you need to be ready for that, for pushing past the common sense part of your brain and going with that small part that just wants to finish without walking the rest of the way.
20 miles will leave you with maybe another hour of running, it is a lot to deal with.
If you can't push past 8 miles marathon day is going to feel like the longest day of your life.
I came back from a 6 month lay off from an achilles injury by running half a mile and building up from there. Better to go slow and be safe, don't want to be out for another couple of months for the sake of pushing yourself.