becky are you planning to run with the run/walk pacers at FLM? If you are than I think they aim for around 5.30, which means the running bits are done at around 11 min miling.
I completed one marathon using run/walk, I did 20 mins running, 2 mins walking and finished in 5.28. I did this because I was returning from injury and hadn't done enough long runs to run the whole way.
Long runs need to be practised using run/walk method, I ran no stop for all other runs in the weeks training.
Good luck.