You'll be fine. Recommendations to get some saddle time in beforehand are wise, but manys the healthy, fit 23 y.o. that headed out on a tour with no particular preparation.

Nearly everybody rides south to north, taking advantage of prevailing winds. It will rain. The toughest days (hills) are in Cornwall and Devon - the first days of the ride if you're going S -> N.

There's no 'official' route. The Sustrans guide is mostly good (suggestions on local brews and eats - wonderful!) and the Cicerone is popular. "End to End Cycle Route - A Safer Way" looks spectacularly bad. Folks have mapped routes to minimize or maximize climbing. Sections of the National Byway can be included, and some routes touch the Hebredes and passage the Great Glen.

Trains serve Penzance (of Pirates fame), just a few miles from Lands End. Trains serve Thurso and Wick to get back - make a cycle reservation on the train. My suggestion is to take the ferry to Orkney and ride on to Kirkwall, flying back on LoganAir.

Cycle UK's forum on LEJoG:
https://forum.cyclinguk.org/viewforu...f1edf61b77c56b