I think the Andrea part was the weakest part of the book because not enough pages were spent on the storyline. Its ok as it is, Parker wasn't writing the next great American epic. I kind of get it though - Quenton and Andrea dated for all of one semester, so its not some super long term relationship. They dated from September to January in Once a Runner. Its implied that Quenton breaks up with Andrea too. Andrea could easily have not even appeared in Again to Carthage without the book being any weaker.
Anyways, if you want a book where dating plays a more central role to the story, there is Pain. It has its own problems, the author seems to think alcoholism is cool and that "just a 6 pack" is a light night of drinking, but the protagonist's relationship is just as central as his running.