I can understand how Rob did his first few marathons and hear me out on this before you all go running your mouths off.....Of course I wasn't there so don't know the full story but here is my theory for what it's worth. Rob would have done his first marathon as a run just to finish it for his bet. He would have had no idea how it would affect him or not. It appears that it wasn't that stressful so he did his second one faster and so on. This is not a crazy thing and I see it often. People do hold back until they see what they can really do. I know Rob has a preferred speed that he runs at optimally.
And no. Rob does NOT have a "preferred speed" that he runs at optimally.
One of his fellow runners in the 2015 Race Across the USA clearly remarked that every day Rob would sprint the opening few miles like a bat out of hell at 5:00-5:30 pace and then finish in a jog/walking.
And his splits at all 3 of his certified marathons show absolutely no even pacing. At Brighton, he ran an opening mile in 5:23 and yet was running 30 minute 5K splits at the end of the race. Similar precipitous splits at both of his runs in London.
Rob has the most erratic splits I've ever seen by someone who has allegedly done so many marathons. No exaggeration.