You are basing your marathon pace off of 5k/10k times. Do you honestly think you could finish a marathon at that pace?
I agree with Jimmy here. When I did the Daniels plan last season, I also did not fare well with the M pace workouts. They were not comfortable at all. After thinking about it more, I thought his M pace was a little too optimistic, considering I had only been back to running less than a year. Sure, my 5K races were better, but they never translated up in distance. Part of it could be you are more FT than ST.