why did Octavian travel to Lupiae then Brundisium instead of straight to Brundisium?

Is there a particular reason you imagine that he did?

It sounds like this is a conflation of two separate reports. Dion Cassius 45.3.2 reports that he sailed to Brundisium; Appian B. Civ. 3.10 reports that he went from Apollonia to Lupiae, and discusses the point explicitly:

he sailed across the Ionian Sea, not to Brundisium -- for he had not yet had any experience of the army there, and was avoiding all risks -- but to another city not far from Brundisium, off the road and called Lupiae. He took lodgings and stayed some time there.

Appian looks like the more reliable account (especially given that whenever Dion Cassius disagrees with another source, it's often wisest to accept the other source's word).

On the length of the journey: the ORBIS model reports that a crossing from Apollonia to Brundisium would take about 1.3 days (or a bit over 31 hours) in March. It goes up to 1.4 days if you move the crossing to April.