Dealing with low-quality legacy student-work
The same way you deal with mind numbingly boring work in any area: one page at a time.

I've found the Pomodoro Technique very helpful. Set a timer for some short period of time like 25 minutes, and work without a break. Then take a short 3-5 minute break and repeat. It's like swimming. You go underwater for a period of time and then come up for air for a short break when you need to. (n.b. I have very little experience swimming) 

If possible, try to have other things to do at work and work on each thing every day. This way you're not stuck doing the boring stuff 8-10 hours a day. If you do this, it's helpful to try and get as much of the crappy work done at the start of your day. This way you're not dreading and putting off the tedium all day.

Also accept the fact that some days will be more productive than others. If you set a goal to do X number of pages a day, you can easily get stuck in a spiral of unproductivity and procrastination that bleeds over into the next day. Do as much as you can, but take breaks and try to keep your spirits up.