Just as marathon runners have a series of barriers and comparatively easy bits during a race, so, too, do writers. Though every book is different and some are much easier than others, I have found, writing non-fiction that requires research that the below has a rough validity.
0-30,000 words is pretty easy.
30K to 50K not too bad.
Around 60K there is period of resistance; you break through if persistent.
Then it just gets harder. But still bearable until you get to 90K- around 300 pages of a normal sized book.
The 10k from 90 to 100k seem to last forever. The wall.
Just under 100K it seems ridiculous you could write more. You go on.
At 102 or 103K a great vista opens: huge relief- you've broken through and it seems as if you could keep going for ever...