From your post I don't know how experienced you are at cycling, but the distance from Poole to Falmouth is approximately 144 miles, so for a start I would say try doing it over a least 2 days. Even taking three days or more would be OK. The route also has some hills that could slow you down a little. For a journey such as that I would suggest some padded cycling shorts would ease the wear and tear to the bottom area. Sweat is very salty and could cause a rash in the crutch area, so over a distance such as you're planning a spare clean pair of shorts would be a good idea. There are many proprietary creams that can be used to reduce the chance of getting a rash but personally I use Vaseline and it seems to work for me!!!!!!!!!!!!!!!!!!!!!!!!