Two days ago, I witnessed a man run for 13.1 miles with a cast on one hand and a three-foot flag in the other. The man jogged along serenely, carrying on lengthy conversations while wearing such a strangely plastered-on smile that you would think he was trying out to be an alternate for the Patriots' cheerleading squad. Who was this happy-go-lucky, insensible-to-pain, flag-waving, crazy man? He was, pure and simple, a runner.

He is volunteering as a pacer for the race, running at a steady 8-minute-mile trot as he completed the course. He was actually a marathoner who had qualified for the Boston Marathon this year but had to begrudgingly relinquish his number when his doctors forced him to have knee surgery four days before the race. He actually asked his doctor if there was a procedure with less than four days of recovery time!

An estimated 423,000 people completed marathons in the United States in 2004, according to Runner's World Magazine. After I ran my first half-marathon on Sunday, my mom asked me over the phone whether I had that "euphoric running feeling" that serious runners describe. The short answer-absolutely not. After running 13.1 miles, all I really wanted to do was sit down.

So what is it exactly that induces us to run like mad?

When I decided to start training for my first half-marathon last semester, it seemed to me like a natural progression from my normal workout regimen. I felt that the hours spent staring blankly at a dot on the cinderblock wall as I trained on a treadmill would pass quicker with an actual finish line to shoot for.

The training went fairly smoothly. I got to know the 7 a.m. regulars at the gym, I learned a lot about Waltham and Newton geography, and I got in fairly good shape while managing to avoid any horrendous injuries. Despite having to contend with my fair share of blisters, bruised toenails, chaffing, whistles and "hey baby's," I felt pretty good heading up to the big day.

The people I ran with in my first half-marathon were the first serious runners I had a chance to speak to about their training sessions. Sue, a nurse who works 12-hour night shifts, told me that she would try to get in her long runs on the weekend and fit in as many three and four-milers as she could during the week between sleeping and work.

In February, Runner's World Magazine ran an article about a marathoner who ran 350 miles in three days as he trecked through Northern California without stopping. The trek took him 80 hours and 44 minutes and he sustained himself by scarfing down burritos and pies handed to him from his family as they chugged alongside him in their RV.

A sign posted on the lawn of one of the houses we passed on our half-marathon course said that the owners were "A pair of drinkers with a running problem." Runners, it seems, are born with a glitch in their hardwiring that actually causes incoming pain signals to trigger an inhuman-like response to keep running, to push harder. For me, those pain signals tend to activate an instant mental list of all the reasons I should quit my run early.

So people run for every reason under the sun: Exercise, family, camaraderie, etc. But for me, completing my first half-marathon was not only a chance to test myself; it allowed me to feel like a superhero, even if only for a day.