Hi Jon
2 of us have just finished (Thursday) a 9 dayer, 881 miles in total, pretty direct.
We went unsupported but carried very little, staying in B & B's along the way. I had a max weight of 3.5kg's.
I'll pop the route and B & B's on soon and had them all pre booked. Our shortest day was 75 and our longest 122 (got lost after the Forth Rd Bridge - in torrential rain!!)
I started cycling in mid April and just before I left I had accumulated 2500 training miles, the lad I went with did less but did better 'quality' training, he worked on power and speed sessions when I did distance. By the time I left I had done back to back 100's with weight which I found served me well on the challenge.
As others have said it really depends on the number of days you plan to complete it in. If say 9 was your target the first few days will be challenging but you definately settle into a pattern and your body becomes accustomed to the mileage. By the time I got to JOG I felt as if I could turn round and go back down, your body adapts amazingly well.
Eat lots drink more (water!!) and keep the pedals turning, sounds simple but there will be times when you'll feel low but grind on. We went LEJOG instead of JOGLE for the prevailing winds, we set off from LE into a 20mph headwind which persisted for 3 days

and the riding was really tough.
'User3143' has said before 'keep your head together going through Cornwall and Devon and you'll certainly finish' this is the best advice to heed, I thought about it lots when ploughing through the headwinds!!
After Devon the riding is much more sedate up to Ludlow through Chepstow, nice riding and you can make good progress.
We did not find any hills that worried us at all, we actually got to Penrith waiting for Shap when we had already been over it

It seems to have attained legendary status but it really was nothing (sorry if this sounds arrogant) but that's what we found.
By far the hardest point was reaching the climb up to Bristol Airport after circa 88 miles in the hot sun, that was murder! Dropping into rush hour traffic a little 'bonked' made life pretty difficult.
Scotland no problem either not as bad as Cornwall by any stretch, lots to look at to keep the mind occupied. Cornwall is tough due to the relentless nature of the hills, they're not 'big' but they come at regular intervals, if you stop at Okehampton you miss the mother of climbs after you've done around 100 as you turn off the A30 just before it.
Really, set your training up depending on the length of the challenge and go for it - oh and pray for good weather, we had 1 day (122 miles from Edinburgh to Newtonmore in storm winds and relentless rain) out of 9, 8 lovely days of sunshine in September, very very lucky!!
Good luck.
Robbiep