I'm a fan of the idea of the NCN but I think the current implementation is flawed and this sums up why, really:
The longest continuous NCN journey I've made is probably just 31 miles, Holkham Hall to King's Lynn. If I've gone further south on the same journey any time, I would have left the NCN to cross King's Lynn on more direct cycle tracks and quiet streets, rather than do an inverted L just to push through the 400m no-cycling section (which was suspended for a few years but I don't think I cycled through as part of a long ride during them). Heading any further east than Holkham would be impeded by the unsurfaced detours it makes - even if you're happy with gravel, there are shorter/easier ways east than the way the NCN goes.
Westwards, I've only cycled on NCN as far as Wisbech and only once on the NCN route because it makes two unnecessary awkward detours in just 10 miles. Any further west than Long Sutton is reached much more easily by cycling directly to Long Sutton and then joining the NCN.
Southbound, I've always avoided the detour and sand in Downham Market, so that's only 14 miles on that section. Even if the sand gets replaced by parallel tarmac alongside a housing estate in the next year or two as expected, I might do the town centre detour to go further only on NCN, but it'll still probably max out at 51 miles for the forseeable because the route to Cambridge is still not complete - even then, I'd be tempted to cut off the Welney and Wicken wiggles to shorten it by 4 miles.
Where I used to live near Bristol (home of Sustrans!), I think the longest NCN ride I did was maybe 10 miles. I have ridden a route which has since become part of the NCN but even that's only 14 miles, although it goes the opposite direction from the same start point.