Why am I doing this?

So I've decided to run a half marathon in 12 weeks time in Auckland. I even went to the gym and I have  a programme and everything. It tells me how long/far I should run each day and what drills I need to do etc. [Apparently I run like a girl and not like a girl-runner. My gym instructor tells me there is a difference]. I'm not looking forward to the 18km training run about week 7.  =/

I keep asking myself why am I doing this. I'm not completely sure, but so far I've come up with the following:
a) I have nothing else to do over summer.
b) My friends are doing it.
c) So when I go to house parties and dance, people won't think I've gone for a swim when it's actually just excessive sweating. (real story)

I'm sure as I get more into it I will have to come up with some more reasons to keep myself motivated. But these will do for now.