Good session there Carl - I must say you seem to be progressing well in terms of both improving your speed/pace AND your endurance (some impressive LSRs already in the bag)!
I seem to showing some progress in terms of speed but feel I need to work on the endurance/LSR side of things... no LSR last week because of illness and a disastrous "bonk" near the end of the previous weeks 16 miler.
Ricky - endurance is definitely the key for me and I guess this is what will count in trying to get the pb.
As my marathon is early April I have hit the 20 milers earlier than others with races in late April / May.
In my previous 2 attempts at the marathon I had only 1 20 miler in my plan and looking back on it now I did not do them very well. This time I have now got 3 under my belt, have another one with 8 miles MP planned for next weekend, a HM the weekend after and then spoons has me doing a 22 miler with 12 miles of MP followed by a 20 mile SLOW before going into my taper.
Now when he suggested it I thought it would kill me.
But I think the mixture of the tempo and interval sessions (which are tough) is what actually helps build the endurancve in these long LSR's.
Also my training plan has emerged as I have progressed. I never expected to run 6 20 milers with a HM thrown in during a recovery week.
But I am pleased with where I am at currently and trying not to gety ahead of myself. As you know this week I had to adjust my schedule and in a funny way it has helped me.
So it is natural to worry about the endurance but it will come. Keep putting in the quality sessions that you are and as the LSR's build up over the weeks you will amaze yourself. 
