Do you want to go east or west of the pennines? The former is longer but I think much more scenic, the latter, still scenic but you have some major conurbations to contend with on the way. Also regarding 'training' (or whatever you want to call it), think about the number of hours you'd want to spend in the saddle each day (e.g. 6-7 hrs would be a reasonable time). When you prepare, do plenty of training rides of that length of time, and do it until you recover well enough to get up and do exactly the same the next day. If you're touring with a load, you'll probably ride at around 12 mph, maybe more, thereby enabling you to cover between 70 and 80 miles per day. That's plenty even if you want to do the long way round.
My lejog was around 1,100 miles and daily distances varied between 54 and 84 miles, depending on how we felt. We were riding for 14.5 days.