NN
Let me have a go - but bear in mind they may well be opposing views.
P&D tell you to run your LR at MP +20% accelerating to MP+10% for the last 5 miles (p16).
For me that usually coincides with 70%max and 75%max HR - and I don't get much drift at those low HRs (so the increase to 75% is due to increased pace, not drifting HR over time). But you may well be different to me, so....
Problem - you don't know what your MP is going to be. So we're in to rules of thumb
eg
(1) Female Mara time average = HM x 2.25 (male is HM x 2.29). There was a Moraghan thread on this - correlation was very high on the RW test data, and was true across all ages groups...ratios didn't get much better until you got to the sub-3 guys
(2) 5 x 10k time?
(3) I use Hadd, who makes you do 10 mile runs at 80% to 83% max HR - once you find a pace that you can maintain in that band, with no upward drift, for 10 miles, then that's your MP according to Hadd. (Over time Hadd has you nudge up the intensity but I find my pace increases at that HR over the course of a campaign anyway...)