Hi Charles,
Yes its a Centurion running event. it was well organised and most of the CP staff were either ultra runners or ultra race directors themselves and the experience showed. I imagine the SDW will be as good.
The canal is quite well marked ... just follow the water
the real trick is to ensure that you are on the correct side and mark all the bridge crossings. there are usually plenty of bridges but even backtracking 100yds is immensely frustrating after 70miles or so. (there is one crossing at about 50-70m that I never remember to take!)
The other one to watch is where different arms of the canal come in/go off, like at Braunston and at GJA. it can be easy with your head down just to wander off up the wrong arm and sometimes its not easy to work out how to get where you want to be. again check out where these junctions are on the map in advance.
I would recommend trying to get hold of the GEO projects maps, which are pretty useful - though the scale gets a bit frustrating and seems to alter as you approach London. It seems that the scale reduces just so they can fit it all on. as a result it actually does take you longer between bridges than earlier on in the route and is not just tiredness that makes it seem like that.