The Torah our Forefathers studied and lived was not the Torah that we have with the narratives and the "stories" of what happened to the individuals mentioned in the Torah. Rather, it was the blueprint for all Mitzvot. It was so extensive that the Talmud actually relates that Abraham kept all the Mitzvot even the Rabbinical ones! How did they know what to keep? The Talmud explains that they had an inner understanding of what it is that God wanted from them.