Several questions arise.
There was no statutory sacrifice in the evening, only the burning of what was left over from the day, according to which Maariv was fixed. The conclusion of the Gemara is that we follow the sacrificial times. Fixing the times according to the sacrifices also allows for the inclusion of the Musaph, also a sacrifice, whereas fixing the ruling according to the Patriarchs would not allow for Musaph as they only had the three periods. From Isaac we learn something of the quality of prayer, according to the commentary Haketav Vehakabbalah. Isaac went into the fields to walk among the bushes to clear his mind of his everyday thoughts and occupation and thus be able to pour out his innermost feelings.