Taking the train home

Here is an interview question that a friend of mine was recently asked on route to a job in the city:

A man leaves his office every day at 5:00, takes the commuter train and arrives at his hometown depot at 6:00. His wife arrives at the depot at 6:00, in their car, and drives him home. This is an everyday procedure. One day, he leaves his office at 4:00 instead of the usual 5:00, neglects to inform his wife, and arrives at the depot at 5:00 instead of 6:00. He decides to walk home. On the way home he meets his wife, who is driving to pick him up at the usual 6:00. He gets in the car, they drive home and arrive 20 minutes earlier than usual.

Question 1: How long, in minutes, did he walk? Question 2: What un-stated assumptions went into your answer to Question 1? What could go wrong?