I had the same problem and I was able to solve it by timeboxing reading as if it was work, even though it's a hobby. I get comfy, but on some ambient music, and set my little desk timer to about 40 minutes. It's set to not beep when time runs out. I'll start reading, and for the first 20 minutes or so I'll still feel kind of antsy and keep looking at the clock, but 9/10 times I end up finally getting engrossed in what I'm reading and I don't even notice that the timer ended. By then I'm in the groove and am good to read for a couple of hours.