Obviously there are millions of reasons to exercise (besides just fueling a competitive drive). Some new research even tells us how it fuels the brain. I wouldn't have gotten a master's in the subject if I weren't completely fascinated with the awesome benefits to every body. Plus it helps to have a little knowledge when you want to get faster. :)
With enough research and hopefully enough common knowledge of why exercise is good for the general population, I won't even start there with my rebuttal to Ms Sedentary Observer. So let me start with dissecting her question. Why do I feel the need to do so much exercise? This part is pretty easy for me. She has no idea how much exercise I do now, and even less an idea of how much I used to do in Ironman training. Seeing only a couple of hours a week of my 7-8, I wonder if she would be overwhelmed at the thought of exercising over an hour a day. Ever. Dr. Clapp's research subjects were considered in the "stopped exercise" category if they dipped below 50% of the usual pre-pregnancy training. And many of his subjects did well over their usual 100%. That is not me, but I can't see how many former long distance triathletes can keep up the motivation to do that while lugging the extra person around. Or get enough calories to grow said person. Rebuttal: You think THIS is so much exercise? I feel lazy compared to my training husband.
Next part of the question: Why do I feel the need to do so much exercise? Ever heard of the enjoyment of physical activity? How do you explain this to a sedentary person? I actually enjoy exercise more now than before pregnancy. I used to wonder how people could feel good after a workout when I'd drag my tired self home and plop on the couch for a few hours after the more strenuous sessions. But now, I get it. I feel better after exercise! More than just that lack-of-a-negative-stimulus thing too. Rebuttal: It's not a need, it's a want.
And saving the best for last: Why do I feel the need to do so much exercise? Let me break this one down even further. I know a lot of research has been done since she had children (who are my age), but either keep up with it, or keep your mouth shut.
My baby is getting benefits from this exercise. He is not being bounced too much, or starved. He is getting a change in blood flow when I exercise, which is making him stronger. A baby who is stronger can withstand labor stress much more easily, which hopefully will decrease the risk of a cesarean. I worked hard on developing his placenta to be bigger in the first few months (via exercise), and now that's paying off. Even with decreased blood flow, he still gets sufficient nutrients. Recently, some researchers found a dose-response effect of exercise on the heart rate of babies (link and link with a graph). The more the mother exercised, the lower the heart rate of the babies they were carrying. My baby's getting fitter before birth.
Moving on to the secondary person who benefits, me. Hopefully I will have a shorter labor and less discomfort throughout pregnancy. I must say I've been relatively comfortable, except for the head under my ribs and nightly heartburn. I have no signs of swelling or cankles-- I probably shouldn't jinx my last 4 weeks-- YET. My weight gain has been moderate so far, but steadily increases, just as it should.
After the baby is born is when the really exciting benefits should show up. Not only have I been training all winter with extra weight (isn't winter the best time for building strength?), but I will be losing at least 6-8 pounds of baby plus several of placenta in a few weeks. How light I'll feel! And the best part is the 5-10% increase I may see in my VO2max. What triathlete wouldn't pay to get a boost like that?
As I see it, this is my chance. I can not only grow a healthier baby, but reap the rewards in my athletic life after. With all that going for me, how could I not feel the need to do all this exercise?
A better question would be, why do I feel the need to eat so much ice cream? I can't answer that one.