I was there in January. I regret not making the time to get to Herculaneum, as it sounds spectacular. Covering all the open parts of Pompeii in anything less than a full day would be infeasible, so I'm happy we allocated the whole day there. And yes, it's an exhausting site - fairly flat with gentle slopes, but stones & scale conspire. (I walked 16.5km that day.) I'd also suggest unless you're likely to return, rushing through such places means you miss much of the impact.