How long is the walk from Santa Lucia train station to Ca'd'Oro?
Google maps shows the distance 17 min walk at 1.3 km:
https://goo.gl/maps/2roRKN7Dz5op4uNy8
That is a fairly easy walk up one of the few wide “streets” in Venice (most of it named Strada Nova) with no meandering around in narrow passageways. But it can be fairly crowded with pedestrians.
You will have 5 bridges to cross, each with steps. Or you could just suck it up and spend 7.50 EUR and take the No 1 vaporetto to the Ca' d'Oro stop.