I think booking as one goes could be difficult this year, as (I live in Cornwall) the word is that demand's massive for the post-lockdown period.
One suggestion might be to try to book in advance around the Penzance/St Ives area for several-day blocks. The bus services round Penwith and Mount's Bay are very good, so it would be easy to bus out and back from the path to a base.
As far as difficulty goes, the north coast of Penwith can be very tough going, as the path is so uneven and rocky. The Lizard is much easier, particularly on the west side, so I recommend that stretch as a good one to begin on and get into one's stride. In my experience, it shouldn't take many days to shake off unfitness, so long as one eases oneself in sensibly.
I don't know which bits you've done before, but the part from Hartland Point down to the Camel estuary can be very rigorous, though also sensationally beautiful. From Padstow down to Hayle is easier going, in my opinion, and another good stretch to run oneself in on. The south coast is generally less tough too.
Good luck.