It is a phenomenon that many runners talk about, and one that makes quite a few a bit agitated: the runner weight gain. It can happen for a number of reason, too. If you start out your running career with little to no previous exercise history, (and thus, not much muscle mass built up) then the weight gain you are experiencing might actually be a desirable weight gain, as your quads, hamstrings, calves, and glutes are getting stronger and more built and toned. If you suspect this to be the case, try staying OFF the scale actually. Instead, measure how your body is changing based on how your clothes are fitting, and how your running is improving. If your weight gain is because of muscle gain, then you will start noticing your clothes becoming tighter in "all the right areas," and looser in others. Additionally, you will start seeing yourself be able to run faster and longer.
But then there is the weight gain that is not necessarily just muscle, but some fat too. Perhaps this is actually a GOOD thing, as a lot of runners are actually underweight and need to gain fat to be healthier. The main reason for this type of weight gain in runners is that, first off, running makes you very hungry. And the more you run, the hungrier you become. Running, and other cardio vascular exercises, rev up your metabolism and keep it sustained for longer periods of time (even after you have stopped running and exercising). Furthermore, if you HAVE gained muscle due to your new workout plan, that will cause our metabolism to rev up even more, which means you are burning through more calories at a faster rate in order to create energy to sustain you. This means your body is screaming at you to put back some of those calories because it is so hungry. A lot of runners are not used to the extreme hunger they experience during their training, and especially when they are training for intense and longer races and, thus, are logging a ton of miles throughout the week. Furthermore, when the body is tired and when sleep is put on the 'back burner,' the body's natural response sometimes is hunger because it needs rest and energy and if it can't get it through sleep, then it asks for it through hunger.