So I was there about 5 months ago. I'll be honest, I'm in pretty good shape and even still, Pompeii is way too much to do in a day. Herculaneum is wonderful and very well preserved, but if you only had an afternoon you could see it with some ease. Were I to do it again I'd likely spend one day in Pompeii, one day in Herculaneum and on the slopes of Vesuvius, and at least a day in Napoli proper. That would get everything in any respect, but would let have a real sense of those couple sites.