Well, is there any way to do some tests before the big day?

The reason why you would run/walk is to get a faster time... right?

I know for me, I did a 5 run/1 walk combo for 12 miles and my time was 6 minutes faster than what I did when I ran 12 miles straight.

That told me, I need to do a run/walk combo. Maybe you can run a few tests?

But yea, I would say try the 5/1 (1/5) combo and see how that works out.

I also know that when I did my last 10 mile run with a 3 run/1 walk combo... my time was better than if I had run 10 straight miles.

Well, good luck and I say train like you race and race like you train.