I just wanted to say, have a great time Sam. :)
Also a little pointer about a couple of things on the itinerary, unless things have changed drastically.
First day.
Castellabate/Santa Maria.
Some cobbles and steps in Castellabate. I would suggest flatter footwear. 🙂 Santa Maria is a nice small town, with some decent shops, and there may well be the option of coming back on the coach to the hotel, or staying on, and having lunch there. You can either walk back to San Marco on the wonderful beach, or get the local bus back. There are some bars in the town, and a couple of restaurants on the beach. They may still be open when you go.
Paestum.
The walking amongst the ruins is on gravel paths mainly. It's quite commercialised there, and will be busy, but again there are some quite reasonably priced shops.
Elea Velia.
Some inclines and declines, but I can't remember many steps. Very interesting place.
San Marco. This is a lovely village with friendly locals. There are a couple of small supermarkets for essentials, a really nice church which is worth a look around, and what I am told is a good restaurant by the church. The ice cream/bar/snack place up the steps from the harbour is wow! :D
If you go to Amalfi and Ravello (and it is certainly worth it for the drive alone, if the weather is good), be prepared for it to be very, very busy. Awesome places.
If I can help more please let me know. 🙂