In 2001, the year before my planned LEJoG, to test whether I could cycle 100 miles a day, I cycled to my Father-in-Law's in Manchester without any prep (I was a non-cyclist in 2001).
It was 200 miles in 2 equal days.
I made it but I was reduced to going up and down stairs on my arse and hands for 2 days afterwards. To say that 200 miles came as a shock to my leg muscles is an understatement.
The following year I did about 6 longish training rides (40 miles to work and 40 miles back, six times) over 2 months and I was fine on a 9 day LEJoG.
It doesn't take much prep really, however no prep at all can leave you incapacitated, as I found out.
Eddie Izzard's achievement is amazing, and the fact that some days he was on his feet for 10 hours makes it even more so.
(A top class marathon runner could "jog" a marathon in under 3 hours)
Getting up in the morning, when you are losing toe nails, and realising you are going to run another marathon, again, needs some serious determination.
Fitness is no use without determination on Endurance events. I doubt that few PBP DNFs are down to a lack of fitness and yet 25% don't finish.