This question is kind of tricky and has many answers. One would expect that because runners are getting exercise on their running days that they would lose weight. The main reason runners gain weight is that they are active for the 30 to 90 minutes that they run gaining cardiovascular fitness, but afterwords are not so active. The body needs fuel for the run, but we runners can overcompensate with calories for the rest of the day. There are a few easy ways to combat this problem.
I do think that it is important to keep a daily food diary. The calories that we do not burn during the day are stored as fat. There are several web sites and apps to keep a daily calorie count. A couple of good examples are the MyFitnessPal app for smartphones and SuperTracker from the USDA for counting calories. These sites and apps not only track total calories eaten for the day they can also track daily macro (carbs, protein, and fat) and micronutrients (vitamins and minerals). I think it is essential that athletes of any level know what their daily intake of nutrients is.
Runners also need to lift weights to gain some muscle mass. Muscle is denser than fat so a higher muscle mass will burn more calories throughout the day. This means that having more muscle mass the body is burning more calories even at rest. Age is a factor here. As we age we tend to lose muscle mass. It is important to lift weight two to three days a week to keep muscle mass from declining so fast as we age. Runners always tend to worry about gaining too much muscle mass when they lift weights. My thoughts on this, if you do not eat and lift weight like a bodybuilder you will not look like a bodybuilder. I like to use dumbbells when I life at home. They are easy to use and there are plenty of movements that we can use as a runner to help gain a little muscle mass. Take a look online for dumbbell training posters. With a quick browse online you can find some nice posters that cost anywhere from $15 to $25. These posters show a nice set of movements for every part of the body. If you are not sure if you want to life at home you can always join a fitness center to get your lifting in.
We may also have something physically happening that needs some medical attention. Low thyroid function is one of the medical problems that can cause weight gain. Other reasons to consider are diabetes, aging (again), stress, water retention, or not getting enough rest. Some of these can be address with medical assistance while the others or totally under our control. We may also be taking medications that have weight gain as a side effect. If you are not sure if there are medical reason go to a local wellness check and have your blood work completed. We should be doing this annually anyways just so we know how things are going.
One last thing. All calories are not the equal. 100 calories of candy is equal to 100 calories of veggies as a number, but it affects the body in vastly different ways. A couple of examples are appetite and energy level. I always say that variety is the spice of life and everything in moderation. Eat a variety of colors when it comes to vegetables. Fat is not the enemy. Do not just grab something off of the shelf because the label say “All Natural”. The word natural as it pertains to labels is not regulated and can mean just about anything. Try to stay away from foods that contained added sugar. The maximum recommended allowance of added sugar for men is 150 calories per day (37.5 grams or 9 teaspoons) and 100 calories per day (25 grams or 6 teaspoons) for women. Eat that apple or orange, but be aware that that bowl of cereal could contain more sugar than you actually want or should eat.
“Man imposes his own limitations, don’t set any”