Hi Rich
I just had a quick look. It looks about 15 ish miles, mebbe a little more. You would need to do a proper check the route to get a more accurate figure. My experience of Cornwall's coastal paths is that they are very much up and down. Nothing too severe,up 200 ft, down 100ft, then back up again. Constant, and very tiring if it's hot.
If you are fit, like many members of this forum, I am sure this is doable. I am not so fit, and if I was walking it, I would have to start early cos it would take me all day, and the ferry staff would probably have packed up and gone home by the time I got there.
Hopefully another member of the forum knows this area better than I and can give you more info.