First day after Lincoln, I'd aim for the C&CC site site at Woodhall Spa, about 50 miles.
Day 2. Woodhall Spa to Fleet Hargate. Leave Woodhall Spa on the NCR1 and follow it until after the bridge over the river Welland. Now follow the coast road around to Sutton Bridge. Then through Long Sutton to Fleet Hargate. Delph Bank campsite is on your left as you enter the village. About 54 miles.
Day 3. Fleet Hargate to C&CC site in the Sandringham estate. Follow the 1 most of the way. It's a short day of around 40 miles but you should/will be able to look around Sandringham house in the afternoon or the next day if you take one off .......
Day 4. Sandringham to Cromer. I followed the 1 until it turned south, then I followed the 30 to Cromer. The campsites at Cromer are on the west of the town so you coming in from the right side. Take your pick. About 50 miles.
I then followed the coast around to the C&CC site at Kessingland. Around 50 miles.
Then I turned inland to the C&CC site east of Thetford Forest. Another 50 miles.
From there it's about 50 miles to Cambridge and a train home for you. C&CC site is to the south of the town.
Or do it the other way around with the train ride first.