[M]ost runners, during their first year, usually hold their own, showing no weight loss.... But runners are actually burning fat by maintaining weight. As you run, you increase the storage of glycogen and water, all over the body, to process energy and cool you down. Your blood volume also increases. All of these internal changes help you exercise better, but they cause a weight gain (not a fat gain). If your weight is the same, a year after starting endurance exercise, you have burned off several pounds of fat. Don't let the scales drive you crazy.I knew the body adapts to endurance exercise by building more capillaries to get blood to your muscles and more blood to fill them. I also knew your muscles get better at storing quantities of glycogen (the form of sugar your muscles burn for fuel). I even knew that that glycogen increases water storage in your body (it's why you lose so much water weight at the beginning of a low-carb diet, and gain it all back as soon as you put a carb in your mouth!). But this was the first time I'd seen anyone talk about things other than fat and muscle representing measurable weight, even though logically of course they must! And I know my strength training this summer almost certainly added muscle weight. I just don't want to be one of those people we all know who eats breakfast off the donut cart and claims they're gaining muscle because they lifted a weight last week! My jeans fit the same, and I still have to belt my pair from last year to stop them from sliding down my hips, so it's hard to believe I'm deceiving myself. My arms are bigger and they scare me sometimes. One time this summer, I jumped at my reflection in the mirror wondering what was stuck to the back of my arm. On closer inspection, it turned out to be my triceps. Since I've been doing push-ups and chin-ups (with a rubber band so I can get more reps) and this kind of inverted push-up for your shoulders where you put your feet on a bench and pike your hips and point the top of your head toward the floor, my back is wider right under my armpits. I had to get rid of an extra-small blouse I just bought in July. I miss the model arms I had where there was an indentation in the back below my shoulder cap, but I figure this is better for me as a person and athlete. I'm hyper-aware of the truism among distance runners that the lighter you are, the better. Look at world champions. Especially, look at the East Africans. There's been a long-standing bias that you shouldn't carry around "extra muscle weight." However, recent research shows strength training helps runners to an unexpected extent. Now the advice is to "gain strength without gaining weight." That's a nice thought, but it doesn't sound very realistic. My training paces are faster and my workouts are easier since the strength work. Tonight I did four miles in 30:45, an average pace of 7:41. That would have been impossible last year! By all rights, the four or five pounds should be slowing me down noticeably, a fact of which I'm extremely cognizant. But they don't seem to be.