In reply to notaclue:
I live in Bodmin, not far from St Austell.
Even if there were only the coast path for off-road running, you' could have a great time just doing that. You can get quite a lot of ascent in, if you want that, in some parts, and can often either do out and back runs along the path, or make a decent loop via inland paths and minor roads. I have done many loops like that don't go on the roads at all.
Apart from the coast path, though, there is also Bodmin Moor not far away. which is a wilder, more fell-like environment in which you are often off-path, never mind off-road.