The RW plans are fine. I mixed one RW with another plan and the two together were fine.
Remember any plan should be flexible anyway, not a prescription ... If you follow it to the letter, it does not mean you will be able to do the marathon in particular time.
So Reading was your first half recently, and Liverpool your first marathon?
I did Liverpool last year in 3:48 off a 1:39 half - also at Liverpool.
To be honest, I think you are being a little optimistic time wise.. aim to finish first, then maybe go for sub 4.
I get you doing about 3:55.. And Liverpool is a little bit harder due to the 3 hills / incline areas... Maybe cost you 5-10 minutes?
But be flexible, 35-45 is fine mileage wise for me, did 3:30 off that. Everyone varies.