In reply to TheDrunkenBakers:
If you go on the north Cornwall coast, then once you go much further north than Polzeath it gets very hilly, so the running gets tougher and the supermarkets get further away. It is beautiful though.
Polzeath, though not itself in any way a chocolate box, is quite a good centre for what you want. A massive beach, walks/runs either side along the coast path, hilly stuff to one side around the Rumps headland, dunes to the other. Just over the river, a short ferry ride away, you've got Padstow which is full of Rick Stein emporia, another beach at Hawker's Cove and another peninsular you can run around. Supermarkets 15 min away in Wadebridge.
Further down the north coast is Crantock, just across the Gannel from Newquay, is also nice. Again it's on a kind of headland, with beach/coast path/common walks/runs all around. Supermarkets 10 mins away in Newquay.
Further down still is St Agnes. Fantastic coast path walks/runs in both directions. Lovely beaches at Trevaunance Cove and Chapel Porth. Nice pub at the Driftwood Spars.
Or you could try Portscatho in the Roseland , on the south coast. This is a quieter area than the above. Portscatho itself is beautiful as are the runs/walks around St Anthony's Head. Nice pub, plus the Hidden Hut. Not sure about supermarkets.
Finally, Fowey is an idea, if you like that kind of thing. With Polruan on the other side of the river, you've got splendid walk/run options on both sides.
Just a few ideas in addition to those given above.