Gary - running round the whole of Mount's Bay would be pretty scenic, especially "out & back" cos it all looks so different each way! it's an easy cycle ride (which I do early morning sometimes but I stay on the road) and would give you a 12-15 mile run, there are various tracks which will keep you off parts of the road, eg. going over the sand dunes into Marazion
PZ Stn to Mousehole via the harbour, prom, Newlyn Green, Newlyn Harbour - approx 3.5 miles (out & back 7 miles)
PZ Stn to Marazion Bridge (Station House pub & food - plenty parking right by the beach) - approx 2 miles, but if you continue into Marazion, it's another mile or a bit more if you run to the top (out & back 4-8 miles)
The waves sometimes come over the path where it goes along beside the railway line by Pz stn & along the Prom - can be seriously impassable in a winter storm!!
you can choose to park in Mousehole, Penzance or Marazion or many places in between - and it's worth running right up to the top of Marazion, to get the views coming back (slight climb up through Marazion)
I would probably choose to park at Marazion Bridge which would be the easiest to get to from Redruth, straight down the A30 & take the turning to Marazion and just park up when you get to the sea!