An intriguing post from RogueClassicist about the Romans driving geese from the north of France to Rome, and taking around a 100 days to do this - the time taken seems about right, as for example we know that when Sigeric followed the pilgrim route between northern France and Rome, he took around 80 days to make the journey.
Geese being less disciplined and needing to graze on route would take longer, but equally the goose herds, who would have been experienced at making the journey would probably have had a longer travelling day than Sigeric. It's also a reasonable supposition that the gooseherds who probably camped up with their animals probably followed an established stock route with known campsites and water sources and grass for the geese to graze on between hikes.
I also remember - and this has to be anecdote rather than fact as I can't remember the details - when I was at the UWIST field centre in the mid eighties there was someone looking at the history of goose droving from mid Wales to London in the eighteneth and nineteenth centuries before the advent of the railways. The distance would have been around 200 miles (300km) and if they were making around 10 miles (16km) a day they would not be travelling much more slowly than Sigeric. The distance travelled also quite nicely fits with Pliny's report for 100 days for goose drovers to make it from the north of Gaul to the markets of Rome.
The other nice thing about using Welsh goose droving as a model is that they of course drove them across the Welsh mountains, not quite the Alps perhaps but showing that it is possible to drive geese across mountainous wild country, the key perhaps being the use of dogs to both keep the geese together and protect them.