For my 2 pennys worth Gaz - I think for a general base build, what you have done - 10mpd is great. However as a build for a Marathon, you now need to build the resiliance in the legs to cope with running longer. A 20miler is much more tiring than 2 days of 10miles. I think a LSR (approx 20miles) and a MLR (approx 15miles) should now be your priority and do 5miles or so rather than 10miles to allow recovery for the key sessions.
After base building for the past 6 weeks, my general week looks like:
1 x 20m LSR
1 x 15m @ 70 - 75%
1 x 12m inc 80% (building from 40mins up to 90mins)
1 x 11m @ 75%
2 - 3 5m @ 70%
So some days easier than average, some harder, some shorter and some longer.
Edited: 17/12/2012 at 12:11