Instead of assuming you can get eight or ten hours of work done in a day, it’s better to look at your day planner or digital calendar and find the workable chunks of hours.
You might find you only have five hours to work today, or maybe thirteen hours. As soon as you know that you have three hours before lunch, and then after lunch until your 5:30 pm meeting, and then two or three more hours after dinner, you know that you have ten “workable” hours in that day. With that info, you can plan accordingly, work harder and faster for that period, and not be disappointed that you didn’t complete a mountain of work that you did not have the hours to possibly complete.