Steve Pavlina :

Perhaps you try to work a solid 8-hour day without succumbing to distractions, and you can only do it once. The next day you fail utterly. That’s OK. You did one rep of 8 hours. Two is too much for you. So cut back a bit. What duration would allow you to successfully do 5 reps (i.e. a whole week)? Could you work with concentration for one hour a day, five days in a row? If you can’t do that, cut back to 30 minutes or whatever you can do. If you succeed (or if you feel that would be too easy), then increase the challenge (i.e. the resistance).