Clive, my 1st marathon was based on 3 to 4 runs pw, averaging 40 - 45. Towards the end I had a couple of weeks in the low 50s. This is why I am interested in the balance of plans. The 1st marthon was 3.02 and had one LR (17 - 22), a midweek longish run 8 - 12, one tempo or interval run, and maybe one easy 4 or 5 miler. It was a make it up yourself plan based on zero experience
For the next marathon I upped the mileage, and the plan was influenced by P&D. The midweek medium long run became 11 - 15, and I added more easy runs to the week. mileage was closer to 60, but I struggled to find time for all the runs. Personally, the extra mileage bought small gains. i think the biggest benefit was increasing the midweek medium long run, and I'm sticking with that.
For me a Long Run, a medium Long run, tempo/interval session + 1 to 2 easy/recovery runs would more than do it for me in terms of fitting it all in and meeting my targets - I'm not particularly ambitious - just want to run well and enjoy it, and leave a bit of time over to run my business!