Imagine a day in which you only had one thing to do, review an article for a journal, say. That would be a light day. You would probably be able to do this one task by reading the article in about 45 minutes, going to the gym and thinking about it a bit as you exercised, and then writing up a report later in the day, after dinner say, in about an hour.
That day, I would suggest, is more productive than a day in which nothing really gets done, even if there were more work hours in that day. There may only be about 10 really crucial tasks requiring more than one hour to do in a week. I suggest you do those one at a time and as early in the week as possible.
As academics we tend to overvalue multitasking. It makes us busier and thus we can feel better about our work ethic, but wouldn't it be better to just get things done?