A superb ride through flat fenland and historic Cambridge and Ely.
The first stages around Saffron Walden are very gently rolling, but once you descend to the valley of the River Cam, it's flat all the way to the Wash at King's Lynn. Cambridge's cycleways make navigating the city easy, and you ride right through the heart of it all past King's College and Jesus Green. The route is planned to continue along the Cam from here, but long-standing land ownership issues have prevented that, so you'll need to go via the village of Lode before rejoining the route (the Lodes Way).
After Ely's cathedral come straight drove roads alongside the drains. The Wildfowl & Wetlands Trust at Welney is worth a stop, particularly in migration season, and has a welcoming cafe. Crossing the impressive lock complex at Denver leads to Downham Market and King's Lynn on quiet roads.