We have to catch our train at 8.45 am- how long does in take from San Marco? Also what time do the Vaporetto's start?
It's an easy half-hour walk, if that was the gist of the question.
The trip takes about 40min and they start at least as early as 6:45am, so you should be fine.
Did this last summer and it took us about 45 minutes from San Marco to station. If you can, get to the station a bit early. As the train originates in Venice you can board and get settled without feeling rushed. Buon Viaggio!