I'd really recommend doing the bit between Whernside and Pen-y-ghent early in the day as this bit could be dispiriting later on. The other sections mostly have excellent built paths these days which at least mean you make decent fast progress even when tired. Going anti-clockwise also means you get to do the steeper bits of Pen-y-ghent and Ingleborough as ascents, with the long steady descent from Ingleborough to Horton an ideal way to finish.
The first time we did it in 12 hours; this came down to 11 and then under 10 hours in future years. I didn't have a car in those days so 10 hours was great as it meant an earlier train back from Horton to York (or pints in the pub!). I always thought it was amazing that it was possible to do such a great (and long) walk using the train from York in a day.
Good luck.