The Wastelands- But my excuse is that I was only like 10 or 11 when the paperback came out and I hadn't read The Gunslinger or Drawing of the Three yet. Still have the same copy and now its well worn but between the first attempt at reading it and finally finishing it was a period of a few years. Was almost enough to put me off even reading the first two books but luckily that attitude went away.
The Talisman- Another one that I read a bit of in my youth and put down only to pick up when I had matured a bit. The first 100 or so pages are still a bit of a chore to get through even at the ripe old age of 30 but the remainder of the book is worth the struggle.
As for The Gunslinger and it being a hard read... Get through the events in Tull at the very least which may just hook you and you may just find the pages flipping by like you yourself were given access to the neat little trick with your fingers that took Roland years to learn when reloading.