I ride to Lynn all the time, so hope the route below is of interest. I prefer quieter routes so my route is so after Cambridge. No issues with surfaces though parts of the Lodes way get a bit muddy in winter (should be fine now). NCR 11 only goes about 5 miles out of Cambridge before ending at Waterbeach, so I'd avoid that.It does magically re-appear on the other side of the river near Lode though!
If you're not familiar with Cambridge, probably best to stick to the main roads from the station. Cycling in Cambridge isn't generally too hairy. So straight on over the roundabout from the station exit, first right onto Tennison Road, then right at the first lights onto Devonshire Road. Then Right onto Mill Road over the rail bridge all the way to the end where it's left onto Brooks Road. Pass Sainsburies straight over roundabout (2nd exit - and it's a nasty roundabout too) then all the way up to Newmarket Road (wide cycle path available) and right onto that (3rd exit at RAB where MacDonalds is). It's a 15 minute ride to here (for me anyway!) There are quieter ways but perhaps a little tricky if youre not familiar with the place.
Now you can relax on Sustrans Route 51 along Newmarket Road - mostly on cycle paths. Pass the airport, go under the A14 at Quy and follow signs to Bottisham. There, you can just follow the blue signs to Wicken and Ely via Lode. (but Instead of doing that which is a long detour, I'd recommend going through the Swaffhams instead. At Swaffham Prior follow the blue sign to 'Lodes Way' to join the NCR 11 route (it's a left just past the Red Lion pub).
It's all clearly signposted from there to Ely. (You do have to lug your bike over a small bridge at Burwell Lode so beware of that). At Ely, don't follow the Sustrans route unless you want to add about 15 miles to your trip. Instead, where the riverside path from Wicken spits you out onto the A142, go straight over it and head north on the minor road to the A10 roundabout near Littleport. Its about 5 miles.
You then just turn left onto the A10 for 100 yards, over the river and take the first right onto a very quiet minor road that takes you all the way to Denver in Norfolk. At Ten Mile Bank, 4 miles before Denver, you rejoin Sustrans route 11 and just follow it through Downham Market, all the way to King's Lynn through Wimbotsham and Watlington.
It's a very pleasant ride, can be a beast with headwinds though. Takes me about 6 hours to get to Lynn from here (Swaffhams) on a fully loaded bike and I'm pretty slow too.