This event is exactly what it says on the tin - A Marathon preparation event.
You get a choice of either doing 16, 20 or 24 miles and is 4 mile laps around the big Dorney Lake rowing lake used for London 2012 Olympics. The organisers explained to everyone that they could stop at anytime they desired if they had signed up for something longer but wanted to finish earlier.
The entry fee is cheap enough and due to the nature of the event being just to prepare for marathons, a basic medal and a couple of goodies is good enough.
A couple of things that impressed me was that the food/drink stations were loaded quite well with water in cups, jelly babies, bananas, oranges and electrolytes in cups (I think) on offer. The mile 2 water station had run out on one lap around lap 4/5 but was rectified the lap after as more supplies arrived.
There was also free massages on offer at the end albeit with a bit of a queue but fair play to them, they carried on after their designated finish time and did a great job.
One thing that I would say is that you would need to get there seriously early or expect at least a 1.5 mile walk to and from your car to the starting area as parking was pretty tight so the event had a few stragglers starting after 9.30 start time which was delayed 5mins due to people arriving around the start time and still had to get their bib and use bag drop etc. Most cars even arriving early still would have a mile walk to look forward to.
Its a nice flat course so definitely a good event to get a long run in before the big day without any gremlins (hills). It can get a bit boring if laps aren't your thing but overall it was pretty good