Strange, I answered this question a few days ago here: https://clarity.fm/a/2890 but this seems to be a repost of the exact same question. Andrew's comments are spot-on about building a waiting list while you build your product. You want to be able to point to a meaningful uptake of customers...
Joining a top tier consultancy will not help you become a great PM. Full-stop. Yammer has one of the best product management groups and although some of their PM's have left (since the Microsoft acquisition), they still have a great group there. Some of Yammer's best PM's started out in differ...