This was my first marathon ever. The organisation is excellent and support around the course is good, although mostly on the first lap.
There is one nasty hill and steep decent that really hurts the 2nd lap.
The worst bit is the off-road gravel footpath on the disused railtrack. Maybe it's my technique but I had the impression that I was treading water on it. At the end of the first lap, the 2 miles on the footpath were almost 1 minute slower per mile than the first 3 miles of the second lap (on tarmac).
It's worse in the second lap, because you have almost 5 miles on that surface (and you're knackered :-))
Not good for a PB, I guess...