I stand on a giant sand dune in Nags Head, North Carolina, harnessed to an 80-pound hang glider. I grasp the control bar at hip level in front of me. My instructor holds on to one wing with a rope, waiting for the perfect wind. I had anticipated this moment for months, but now I'm scared.

When I planned this hang gliding lesson, it was easy to be brave. A few clicks on the website and I was booked. I had always dreamed—literally dreamed—of flying. Waking up, I'd remember gliding effortlessly, leaning instinctively to the side to turn, aiming my body up to soar. In my dreams I already knew how to fly. I was a natural. I wanted that feeling when I was awake. A hang gliding lesson on soft, forgiving sand seemed a way to experience flight without much risk.

I envisioned myself, lean, streamlined, flying over the sand. But lean was the problem. I intended to lose weight—15 pounds, I decided optimistically—before the lesson. The lighter I was, the farther I'd fly. It was the perfect incentive to take off weight that I needed to lose anyway. I had plenty of time to do it, and a stronger motivation than I'd had in years. But, somehow, I didn't.

Now, waiting on the hot dune, I regret every bowl of ice cream, every floppy slice of pizza. I beat myself up over yet another failure to lose weight. If I couldn't do it this time, would I ever be able to? And more immediately, would my weight make me nose-dive into the side of the hill, like (ouch!) the student I'd just seen? The physics seem simple and unforgiving to me. Heavier objects are harder to get off the ground. I can't reason away gravity.

But when I ask the instructor, a compact, dark-haired woman named Andrea, "Why didn't he fly?" Andrea tells me that he stopped running too soon. He thought he was in flight, extended his legs behind him, and fell flat on the sand. He hadn't built up enough speed. It wasn't his size so much as his timing. She tells me to keep running until I pull the bar under my chin. She'll tell me when.

And the woman who'd had a pretty good flight, then crash-landed? She hadn't pushed the control bar of the hang glider up—they call it "flare"—when it was time to land on her feet.

I ask question after question. When do my legs go behind me? Where do I look? What do I do if the glider suddenly gets too high? Andrea, standing next to me, holding the glider rope, gives each one a detailed answer, and I file it away. Flying is much more complicated than in my dreams.