I did this a couple of summers ago… well worth doing…
my advice…
Just do it, I did it on a bit of whim after my friend suggested it if i had thought about it to much not sure whether I would have done it or just kept putting off.
Get a decent map… I had rubish map at first which meant I was stuck on the big roads which took a lot of the fun of it things improved when I got a better map off my friend.
Do it with someone, I did the first half by myself and the second half with a mate the second half was much better and you can relive the memories more easily.
Go easy the first couple of days, cornwall and devon are very hilly and wore my self out pushing too hard in the first couple of days. I also to travel to cornwall from london and did 100k in the first day.. would recomend it! if iw was doing again I would travel down to penzannce (where the train goes to) one day cycle over to lands end then stay over in penzance on the first night and go from there.
One final peice of advise dont stay in the Inversnaid Hotel near loch lomnad, trust me.

