Charlie,
You are 100% correct ! A runner at my club has worked as a volunteer doctor at marathons. During a very hot event she noticed the elite guys were ok, they just slowed down a bit but had the capability to hold it together. The guys dressed as bananas just walked ! The semi serious guys aiming for 3.nn pushed it in the heat without the requsite level of fitness and got wasted !
By the way, slightly off topic, but related to your comment about a marathon runner joining your training group. Whilst I advocate fairly high mileage for marathon running - it is not the ideal for middle age / recreational health.
I have been using the principles of the GPP video recently. Whilst I have a long way to go in my pitiful attempt at converting from distance running to sprinting I have noticed considerable all round fitness benefits. Amongst other things a loss of 6lbs and a return to my best racing weight of 15 years ago when I was running up to 40 miles per week. Something about the combination of tempo/core/med ball/weights seems to work.
A must for all 50 Year olds !!