This Sunday I am going to wake up at 5 a.m. I am going to shimmy into some spandex, tie my Adidas shoes, fill my backpack with Gatorade and GU energy gel and head to the El.

I will be running my very first marathon.

On October 11, thousands of runners will join me in running the 32nd Annual Bank of America Chicago Marathon. But why do we all do it?

Every time I tell someone I am running a marathon, their first question is “Why?” What satisfaction do I get from putting myself through so much pain? And for that matter, why would I pay the $125 registration fee to do it?

I reply, “It’s fun. I have always wanted to do this.” It’s your standard answer. But really, why do I want to run 26.2 miles on Sunday?

For some reason I still want to do it, even though I know that the last few miles of a marathon could be some of the worst pain I’ll ever feel; my aching, sweating, weather-beaten body will be depleted of glycogen and screaming at me to stop moving immediately.

I may throw up. I will certainly be delirious. Every step I take for the last hour will send searing pain up through my knees and into my upper body.

I started my training back in June. Every morning before work I would begrudgingly wake up before the sun rose to beat the heat. I started out running very little: three miles a day to the Bahai’ Temple and back. And let me tell you this old, slightly chubby girl, who had eaten her fair share of Chipotle burritos and Burger King cheesy tots, was having a hard time making it the whole way.

But the training got easier. I started going faster. I bought new running shoes from a real running store. I also bought one of those dorky-looking fanny-packs you see runners carrying their water bottles in and a bulk–sized pack of berry-flavored GU Gel-an easily-digestible energy gel you can eat while you run. The obsession set in; I did not dread running anymore. In fact, it was the highlight of my day. The runner’s high was my new drug and the supply was free and infinite.

Over the course of these four months, I have bored my family and friends to death with my running stories to the point where they are afraid to bring up the subject with me. I am not even into mile 16 of my war stories and they are looking at their watches, their eyes glazing over, pushing the peas back and forth on their plate.

And now the big day is almost here. The nerves are starting to set in.

I try to remind myself that I have done all the training, there is no way I won’t make it. My old track coach used to say, “Don’t think, just run.” So that is exactly what I am going to do. I am going to blast Survivor’s “Eye of the Tiger” and Queen’s “Don’t Stop Me Now” on my iPod, forget about the pain and focus on the finish line.

A marathon is both a mental and a physical test that tests the level of endurance someone’s mind and body can take.

This type of running brings on a misery and pleasure that non-runners can’t understand. They don’t understand why we get up at 5 a.m. to run under the moon and the stars, or why we spend thousands of dollars a year on running shoes, race watches, microfiber shorts and jog bras.

They don’t understand why we run ourselves into the ground with hamstring injuries and tendonitis, spend months getting healthy and then get hurt all over again. They don’t understand why we don’t mind keeping the physical therapy industry in business.

Am I doing it for the free Goose Island at the end of the race? Am I doing it for the giant thighs and calves I have acquired over the summer that forced me to buy bigger pants? Or because I have an excuse to eat basically anything and justify it with the fact that I ran 12-miles that day?

No. Running is addictive. It’s an addictive, unbelievable experience that will end so sweetly with crowds cheering for me and a medal around my neck.

I run because if I can make it through 26.2 miles, everything else will seem easy. It is a moment in time when an average person like me can feel like Michael Jordan and do something extraordinary.