If you stick to C roads Norfolk (and Suffolk) are great for cycling - I'd guess about one car every two minutes at worse and much less when you get away from the larger villages. Best forget the A and B roads - with no motorways and just the one dual carriageway they can be deathtraps. Navigation can be a bit tricky so good maps are essential.
There are some pan flat bits but mainly it rolls between sea level and 60 metres. No sustained climbs obviously but not as flat as it's reputation. On my rides 40 miles usually means about 1,400 ft of ascent.