To update this topic for 2022 with a report from last year: I did indeed ride the Water Rail Way Lincoln-Boston and it was as good as expected. There also seemed to be a new cycleway under construction alongside the road most of the way from it into Woodhall Spa.
I also rode the Dukeries trail from Fleggborough viaduct to Lincoln, which was pretty good, although gravel at the west end; the cycle route to Newark North Gate railway station from the south, which was pretty and scores points for arriving alongside the old platform, with the station fence now along the island between it and the East Coast Main Line; part of the Grantham Canal (beautiful, avoids hills, but medium-slow gravel); and part of the
Rutland Water loop (surprisingly steep). Oh, and bits of roadside cycleway between places like Peakirk and Helpston and around Stamford which will feature on no-one's top rides list but got some miles covered more easily
I really should use some of these dark evenings to finish the travelogue and post it.