Ran it - first half marathon. Very glad of the trail shoes on some of the downhills; was also surprised by the hilliness, the Garmin reporting 768m of ascent (and a distance of 13.27mi!) - but other than that it was a fantastic experience, really encouraging volunteers all the way round, plenty of aid stations, and the early parts of the course were wide enough that even with 400 runners it was possible to start right at the back of the field and run a 9:00min first mile. I've done parkruns where you wouldn't manage that!
Many thanks to Alan and the ETL/RTL team who seem to be a real focal point for running in the area.
Entrant list for NDW 50 is up and there are a couple of trailers with clips from last year, as well as lots of photos of people crossing the stepping stones (it should be pointed out that there is a footbridge 50m downstream, for those less dramatically inclined)
So my entry threshold for NDW 50 is doing 20 miles of the course - including the Westhumble - Merstham stretch - in under 4.5 hours. That's when I'll feel confident about making it round. See you out there...