The fens were not favourable on the return in 2013; there was a stonking headwind that year as well. It was also 34C in the shade and 38C in direct sunlight. Time was certainly not being made up.
You have to assume that at best you will maintain your contingency, but most likely lose a bit of time, returning across the fens
Interesting... maybe my recollection is wrong. I thought the time in hand plots showed a lot made up time on the way home in 2013... maybe they were just hardier.. or less naïve
My brevet card from 2013 tells me that going south from Edinburgh
I gained 6 minutes to Traquair
I gained 36 mins to Eskdalemuir
I gained 1 hour 7 mins to Brampton (Slept here)
I lost 1 hour 49 to Barnard Castle
I gained 1 hour 40 mins to Thirsk
I gained 47 mins to Pocklington
I gained 28 mins to Market Rasen
I lost 2 hours 14 mins to Kirton (Fens)
I gained 3 mins to St Ives (Fens)
I gained 5 mins to Gt Easton (Fens then appearance of hedges!)
I lost 9 mins to Loughton (I took a fair time out at Great Easton recovering from the day)
So net the Fens southbound took just over 2 hours out of the buffer I had built up in 2013. The faster riders who made it across the Fens the previous night will have fared better, but for anyone in those Fens on the final day, it was far from an easy "let's make up some time" ride in 2013.
I do remember a fair proportion of us pushed on through the night to Market Rasen to leave approx 300km for the final push.
Interestingly the buffer I had at Edinburgh was within 30 mins of the buffer when I finished. So if I'd arrived in Edinburgh with no buffer built up it would likely have been touch and go on the time limits coming back in 2013. Not having a buffer later on can also lead to you choosing not to take enough rest or take time to eat enough which can lead you into a downward spiral.