LORELAI: Why not?
RORY: Because this is Dean’s first marathon. We were gonna go and watch and hang out, he’s totally looking forward to it. I told him about how Andrew gets in a fight with his date in the first fifteen minutes and storms off the floor. I told him about Taylor getting punch-drunk at hour fifteen and telling stories about how he always wanted to be a magician.
If Rory cares so much about Dean’s first marathon, why did she say she would work on the newspaper with Paris on that day? Possibly she and Dean were just going to go in the afternoon or evening to watch (although how will they see Andrew storm off in the first fifteen minutes, if so?).
It’s interesting to wonder why Dean didn’t go to the marathon the year before with Rory (in 2000, he and Rory only began dating in November, so it might have seemed too soon to drag him along). In the previous season around this time of year, Rory was doing the Shakespeare play with Tristan, and she and Dean weren’t getting along very well. It seems so much so that Rory didn’t invite her boyfriend to the marathon.
The Annotated Gilmore Girls 