A few thoughts on Marathon-Pace training.
When I was racing in my 40s, I based my training largely on the schedules in Pfitzinger's 1999 "Road Racing for Serious Runners" -- which were based largely on Daniels' research and theories. But even in his marathon plans, Pfitzinger did not include any marathon-pace runs. He suggested finishing some of the long runs at half-marathon pace, which would be in the realm of threshold pace.
However, in Pfitzinger's 2009 "Advanced Marathoning" second edition, he strongly advocates the value of marathon-pace training, citing the "principle of specificity of training." His 55-70 mpw schedule includes four long runs (16, 18, 16, 18 miles) with 8, 10, 12 and 14 miles, respectively, at marathon pace.
I'm not convinced of the importance of dedicated marathon-pace mileage. Daniels ("Running Formula" third edition), while including marathon-pace training in his plans, says that the ONLY benefit, beyond what one would get from an easy-paced run, is psychological.
My current training is based on the more recent Pfitzinger schedules, and I admit the concept of running 14 miles at marathon pace -- during an 18-mile run -- is very intimidating. And maybe overly taxing. I think shorter mileage at Lactate Threshold would be more productive.
I've never previously incorporated marathon-pace training, and I don't recall ever having trouble dialing in marathon pace from the very start of any of my marathons. My training specifically included ONLY faster paces, and slower paces. But on race day, marathon pace seems to come naturally.
As examples, in my most recent sub-3 marathon, I averaged 6:43 per mile; my first mile of the race was 6:45 (and it had been two years since my last marathon). In last fall's marathon (after 13 years without running a marathon), I averaged 7:51 per mile; the first mile as 8:00, then exactly 7:51 per mile for the next three miles. And I don't wear (or own) a GPS watch or the like ... I just start out at what intuitively is "marathon pace."
That said, I will attempt the four Pfitizinger marathon-pace long runs. Maybe there is some huge benefit I've been missing out on all these years!