I did Wainwright's Coast-to-Coast way back when I wasn't a lardy git, and made the fatal mistake of not wearing in my newish boots enough before I set off. I paid the price after day one - horrendous, bloody blisters.
Fortunately, I had a lightweight pair of Goretex boots in my pack which I wore for the remaining 10 days of the walk (I gritted my teeth and swore a lot, too).
So, huge tip here - wear your intended boots pretty much every day between now and your walk, and wear them on your training walks - heck, I'd even sleep in the damn things, just to be safe.
Smaller tip - if you do get blisters and you've nothing to treat them with, a bit of sheep's wool - just find some on a fence, it's everywhere in the Lakes - and stuff it between your blister and your sock. The lanolin in the wool helps your blisters heal... worked for me, anyway.
Training walks? I wouldn't overdo it. Pack in a few good ten-to-fifteen milers with some hills just to help on the cardio stuff (and with a moderately heavy pack, just to build up some hip and upper-thigh strength).
On the day, drink plenty and go for slow-burn foods (porridge to start, oaty flapjacks for snacks) not instant sugar-rush stuff like mint cake - you're doing an endurance event, not a sprint, and all that sudden rush-and-dip you'll get from sugar-rich foods will leave you feeling lousy.
Most of all, enjoy your challenge - even the bits when it hurts... just grit your teeth, set your sights on a target and keep pounding away.
best of luck!
Steve