I've only been to Belize twice (I'll be back in August!) but in my experience I would recommend taking a flight from Belize City to Placencia if you are staying anywhere close to the airport there. Belize has a very efficient, convenient and inexpensive internal flight network. I've flown from Belize City to Dangriga and from Belize City to Corozal Town (with a stop in San Pedro to get a few drinks at BCs).

I've not taken the drive from Belize City to Placencia but it seems like it would be a rather long one (although with some beautiful scenery). Perhaps someone else here has taken that drive and would be able to estimate how long it takes?

Anyway, most folks on this board would know a lot better than I but taking a flight from Belize City to Plancencia is one other option to consider. Either way you'll have a great trip. Enjoy!

EDIT: After I posted this I realized you said you would be in San Pedro before travelling to Placencia. You may be able to get a direct flight from San Pedro to Placencia, which would save a lot of time. I can't verify this but check with Tropic or Maya Air.