On most cross country routes, I'd expect to ride for 40 or 50 km without putting a foot down.
You might be wrong. Especially since the cyclists on cycle paths in the netherlands have no right of way at roundabouts and most crossings with other traffic outside city limits. And you might have overlooked the fact, that cycle paths on the continent might be on one side of the road only with that side changing every now and then with narrow street crossings at right angles where cyclists have to let traffic pass first. Another quirk making them awkward is occassional raised borders on both sides of narrow paths: No way you can pass someone going slower, you are stuck behind a five year old on his baby trike if fate wills it so until you find a rare spot to escape the funnel.
Audax in the netherlands is often a bit like riding an agility parcours (but so is London, iirc)
When I need to ride in cycle path riddled areas (mind you, over here "mandatory" means that cyclists have to use them) I expect my rolling average to go down by about 10%, so I usually try to avoid them.
That said: If a family wants a nice pootle or the paths give you the opportunity to pass traffic congestions, they are pretty nice. If you can go on long and lonely stretches along canals, rivers and dykes, they are really lovely.