Don't overdo it early and put yourself off. Build slowly over successive rides going up in 5 or ten miles increments every other weekend, carrying a bit of stuff as you go. I did a 50 to Ludlow from Staffs that nearly killed me beacuse I'd never done more than 12 before that..so a bit of a slow build up is a good idea IMO. Surprising how achievable 70 + miles a day is at a steady pace, with the odd tea and cake stop thrown in. You're in the right place for advice, as I was in your shoes some years ago. People like Mick F have a ton of great advice on routes out of Cornwall and much, much more.
I did a recommended CTC route - which did take you miles off course but to some amazing hostels, including Welsh Bicknor and Carbisdale Castle. Carrying a bit extra and camping means you can go a bit more direct I guess. More people involved means more complexity and sometimes more friction..but can be great too.
I did mine on a 20 odd year old Galaxy with some parts upgrades. Great bike and can still be picked up for 3 or 400. My mate used an Edinburgh Coop 'Revolution tourer. Low end and aluminium, but absolutely fine. Mick's done it on a Chopper. For a decent priced steel tourer, Spa cycles are great I think.
Here's a link to the blog my mate James and I wrote at the time. One of hundreds out there that offer insights and alternatives. http://blazingpedals.blogspot.com/
All the best. It's a great experience.
NB. Get a saddle you like, bars you like and some cream for the old undercarriage.