Every time I do one, 110 - 150 miles, once 162, I say I won't do any more. A few months later I am itching to go again. I usually pick a start point, get the train out and walk back. The Humber Bridge is my finish point if I am north of the river, home near Scunthorpe if I am south of the river. Once I did it the other way round, start at the HB walked to Penrith, then train back. I once drove to my friends house in Cleveleys, got the train to Carlisle, and walked back to my car.
I used to do a lot of planning when I first started, but I do very little now. I get the maps, have a look for a possible route, vaguely work out a daily mileage on bikehike.co.uk approx 20 a day, which gives me an idea of places where I might find a bed. I use B & B's, Youth Hostels, and hotels. I collect a few phone numbers of where I might get a bed for the night. If it doesn't work out I just ask someone when it's time to stop.
I'm doing something a bit different next month, driving to Orkney and staying with friends, stopping for a walk on the way, and walking on Orkney. Getting excited about that.
ilona
Sorry, you asked about how far ahead, missed that bit. I get an idea about a month before, then look at maps a week before.