The route is here; through the villages - there's at least one lovely thatched cottage in Rampton and Ramsey seemed smart - then out into the Fens where the roads suddenly become perfectly straight and run along the sides of perfectly straight drainage ditches. Not sure which drains they were, though the one in Ramsey Forty Foot is fairly obvious; I planned this route in my usual way by running through it a few times on Google Maps and writing down notes about which roads to turn onto, and Google Maps doesn't name the drains.
42 miles, which took between 10 and 3:15, but included a leisurely hour at the Dog in a Doublet at mile 35 eating a large steak; having nothing to do in Peterborough, I rushed the last seven miles to get to the station just in time for the 1518 back to Cambridge, so this was purely a cycle ride and not a visit to Peterborough in the slightest. Averaged ten miles an hour, which is good for me.
I'm pondering something in the Ipswich or Colchester direction for my next ride; those are sixty miles, but I think I could manage that. In theory Hills Road extends to Colchester, but I'm sure there's a more interesting route to be found than 47 miles straight down Hills Road. naath warns me that in Essex, as well as villages with names like Helions Bumpstead, there are hills.