All makes sense - although interesting I am being coached for SDW50 & 100 this year (have Brighton marathon in as a day out/recovery run the week after the 50, remains to be seen whether I'll be there) and have regular parkruns in the schedule and lots of interval stuff. Next week I'll probably be doing parkrun the day before the Stinger marathon - coach says blasting 3 miles has no lasting impact at all.
My age has finally led to a decrease in recovery time between hard sessions. I emphasise on the Tuesday, Thursday and Sunday sessions. The weekly accumulation pays it's toll but I just recover in time for the next week. That is the personal reason.
From a training perspective It is about not exceeding lactic threshold pace by more than 30 seconds for the last 9 weeks of the programme. It is The Hanson Method and last year I still managed a sub 7 minute pace for the last mile of the marathon.
As for you a 3 mile run would be a walk in the park. A marathon as a recovery run!