What time are you looking to get round in? We always get some long jog/walk type of days in, at least a couple a month, of at least Y3P distance. We also sometimes do the Y3Ps, and chuck Gragareth in for good measure.
It's also a good idea to get used to being on the hill in all weathers for long periods, regardless of speed. That way you get mentally prepared as well, used to the rough terrain and so on. Lots of the Fellsman is about mental attitude..!