Matt, did you find any challenging hills? I run in Broxbourne woods plenty and wouldn't describe any of the hills there as challenging. A loop of Robin's Nest Hill, Bucks Alley, Ashdene Road, Bayford Road is about as challenging as I can find. http://www.goodrunguide.co.uk/RouteMap/MyRoutes/183107/Footpath. It's much more challenging than the hill profile suggests (completely misses the two sharp climbs on Bucks Alley).