We agree completely. The problem you cited hit me a couple of weeks ago on the LT. I made a plan that was a bit too aggressive from the get-go. I needed to make 12 miles per day over five days, in tough terrain. I exceeded that goal handsomely on day one, but as a consequence, the trail kicked my butt on day two -- enough so that I cut the hike short. There were other factors involved for sure, but the aggressive mileage goal was really my #1 mistake.
I always plan plenty of time to do a hike. I dont like having to quit because of moving slower than planned due to injury, or weather, etc. I usually do finish substantially early. That often means planning 15mpd, then I may do 20-25, or I might do 10 if I want to hang with someone for a day. Being flexible and able to make needed mileage easily is a lot more enjoyable than knowing you have to make max miles every day.
My experience, always shoot for double digit days...........my average is 12. As my pack has gotten lighter, body lighter and hiking plans refined, 12 is very do able, short white mountains, etc.
I think that all of the long distance folks have pretty much proven that it is the NUMBER OF HOURS of hiking, as much or more than your hiking pace. I try to start early, break camp quickly, hike until dark or an hour before. I do not cook anymore so as long as I have enough water to drink I am good to go.
Therefore, I can afford to take a lot of breaks during the day and pace myself so that I wind up at a great campsite before sunset. Ideally at the top of a mountain, which somewhat determines my tentative hiking plan for the day.
As we all know - a mile is NOT a mile on the AT, some go pretty fast, others painfully slow........at 55 with a 30 lb pack, 12 miles is a solid day of hard work physically and mentally.