The trouble with going to Paestum from Positano is that it's a long drive. The ferries aren't running at that time of year, so you either drive along the coastal road eastward, or west and then north through Sorrento to the tunnels, to get to the highway. Use viamichelin.com to see driving routes and optimistic driving times.
Michelin says 2 hours going on the coastal road, but that's if you are going at the maximum. That's only for veteran drivers who know the road and are crazy Italians. If you leave early in the morning, you may have the rising sun in your eyes. You won't want to pass on the road between Positano and Salerno (where you pick up the highway) and you may get stuck in several places waiting for someone to park or pull out, or just a bus stopped for passengers.
From Paestum to Pompeii is mostly (all?) highway. From Pompeii to Positano is a much easier drive, good road (mostly through tunnels) to Sorrento, then a wider (though still 2-lane, no shoulders) from Sorrento to Positano.