Trip I did end of Sept last year I was on 72", a bit too spin-y, so this time I went for 82" which was great. The surfaces are so good that once up to speed it's easy to maintain 30kph, and with even a tiny bit of tailwind, more.
I rode around 10 hours every day between 80 and 190km, just going slower on the shorter days and loved every minute. To me its cycling utopia. I didn't miss hills at all! So easy to ride really efficiently, without losing momentum.
Fietsersbond routeplanner, can't recommend it highly enough. The recreation routes are brilliant, and the direct routes (shortest or racing) really efficient. The direct routes follow main transport routes like freeways and trainlines so cover distance quickly and smoothly, while the recreation routes take in many lovely little paths or back roads that are often deserted. There appeared to be vast swathes of the country without any people, then suddenly there are a few roads and its roadie central. Maybe it helped being the time of year for quietness. Fietsersbond planner just needs "gravel" (yes please) and "brick paving" (avoid) setting to be absolutely perfect. Netherlands' gravel is like any other surface, perfection.
While there weren't any areas I didn't enjoy, I particularly loved the twisty forest and dune paths of the nature areas, east of Utrecht and along the coast, and the east coast of Markmeer has amazing, massive landscapes.
Can't wait to go back and explore more!