Agood runner doing it in three days. Distance running even a 26 mile marathon takes some learning to get right. I fall fowl above marathon distance as my natural tendency when I was running regularly was to go a bit too fast.
It is hard to run or walk at slower than your natural pace, that's why it takes some learning to used to it.
A very good one of our clubs marathon runners (sub 2hr 25min) ran with his wife many years ago in the Mersey Marathon, one of my favorites. His wife ran her expected time and was obviously tired but otherwise fine. However he, running at way below below his natural comfortable pace suffered badly after 18 miles and finished in a heap. He could only put it down to the unnatural pace. I had a similar experience when running well and trying to pace a colleague round the Brampton to Carlisle 10 mile road race inside 60 minutes. I was already running slower that my then 5.30 min mile pace for that route, but kept having to slow to over 6min pace as he kept lagging behind. He finished 30 seconds over the hour mark, but it messed up my running for a couple of weeks.
Dave.