Why run marathons?

Today I ran in the snow. A little like running in sand, but cold. Most people I talk to about my running think I have a mental illness. It just doesn't make sense to them. We'll see if I can explain it.
1. I need a challenge. Hobbies that aren't challenging don't stick with me. Not sure why, they just never have. I have to be working towards a goal or I lose interest.
2. Once you run a marathon you can't go back. It is not like you can get psyched up to run a 5K after doing a marathon. 5Ks are something you do with coworkers for a charity event. They are a couple weeks to get in shape deal. You're always in physical shape to run three miles. It's just getting yourself in mental shape which takes about three running sessions. I'm not knocking them. But they are an opportunity to get out there and show yourself you can still do something athletic and then go drink. Not really a lasting hobby.
3. A marathon is an exciting event to be a part of. From the health expos to the morning warm ups to the post race party. The energy surrounding them is like a pro sporting event or a rock concert. Only you get to participate in it rather than sit in the audience. Also a good excuse for a vacation. I started out running the Disney Half Marathon, moved on to the Chicago Marathon, now I'm shooting for the Disney Marathon. I've thought about putting the New York Marathon and Hawaii on my calendar. Every one is different.
4. It is an ego thing. I'll never be a professional athlete and never could have been. But it makes you feel good about yourself to do something no one else can do. Actually, anyone can do it. But you first need to want to do it, then you need the will to do it, then you need the discipline to get it done. Most people don't get past step one because of fear of steps two and three.
5. I've always preferred individual sports. Running can be done on my own schedule. For me running is not a competition. I'm not running against anyone. I'm running against 26.2 miles. There's a feeling of control when you finish a fifteen mile run in cold weather against the wind with mud clumped on your shoes weighing you down. I can't control what happens in my life. I can't control how others' decisions affect me. I can't control nature. But I can control whether or not I quit. It is me against the pavement, me against the weather, me against my tired legs, me against a cramp, my will to keep going overcoming my will to quit.