Cathy gains inspiration from Lee's passing on the season finale of The Big C. She sets out to celebrate the new year by actually running a marathon, with Paul promising he'll be there - clean and sober - at the finish line/
He has to make this vow after Cathy catches him with cocaine and he explains that he got into the habit, but it's not an addiction and, yes, he must find a new job because he was forced to quit that one due to its drug testing.
Cathy then takes off on the marathon. She struggles most of the way, but is determined to finish. Adam at least receives good news while waiting for his mom: Mia wants to get back together.
Meanwhile, Paul does get high again and crashes and insurance company's party to go off on a woman for denying him benefits.
When Cathy finally makes it to the finish line, Adam, Dr. Mauer and Sean are there to greet here. She also sees the ghosts of Marlene and Lee. Paul then walks up, but Adam comments that dad hasn't arrived yet and it sinks in for Cathy as he falls to the ground and viewers see paramedics trying to revive Paul outside the insurance office: he's dead.