As part of my quest to improve my time management skills, I've started keeping a journal of the stuff I do all day. Its not an exhaustive list, but I try to write down, across various blocks of time, what happened. It's both a tool for study and a spur to action. Seeing my activities written down sure seems to encourage me to get some more stuff on the list. Your mileage may vary. So far this week, I've not lacked things for doing, some of which require multiple hours.
One thing I've observed that applies to me is that after a certain point in the day, a bunch of small tasks is far more tiring (mentally, anyway) than a few larger ones. I've no idea why. Does it feel like more work to have done 6 things in an hour than one thing? Weird.
Another thing is that errands away from the house really eat up time ways you do not expect. The one checker has a long line of people paying with third-party checks from Botswana. You realize that in addition to milk you are in dire need of plastic bags--which are halfway across the store. Or, worse yet, you're picking up pictures at the mall camera store and realize you need dog food, or to return movies, or something else that require another stop. Next thing you know, you've been away an hour for something you thought would take 15 minutes. Yowza.