Well, obviously running will impede you gaining weight, but it's good for other things. Don't know how tall you are, you might be fine the way you are, but resistance training combined with eating more protein (running usually requires more carbs) might help you add muscle, which adds healthy weight. You might want to alternate that with your running.
To gain weight you need to increase your protein intake. I suggest a high protein shake, such as body builders drink. Take it as a supplement not as a meal. This works to lose weight if one is too fat (as a meal replacement) and will cause weight gain for those that are too thin. Contrary to what some might think, weight bearing ie weight lifting, builds muscle mass and increases weight also. This does not mean you need to look like a male body builder, but it does help with weight gain. I assume you do not want unhealthy fat gain. BTW, dietary fat has been shown to NOT cause weight gain for those trying to gain weight. It is just as hard to gain weight if you are underweight as it is to lose weight if you are overweight!
Add high-calorie extras to your breakfast cereals. Bananas, toasted nuts, raisins or cranberries give you good nutrition and lots of extra calories.
Drink juice, Choose juices without added sugar or buy a juicer to make your own orange, pineapple or apple juice. Add at least 1/2 cup of fresh fruit to your breakfast. Nutrient-dense cantaloupe during summer and orange segments will add calories. Add a piece of whole-grain toast spread with peanut butter to your breakfast cereal. Carry nuts and dried fruit in your pocket for a snack or peanut-butter-and-cracker, cheese sticks or power bars, single-serving energy gel packs to eat before running, this will give you quick energy and carbohydrates. Eat snacks and drink energy drinks immediately after running to replace electrolytes. Eat a High Calorie Dinner
complex carbohydrates such as brown rice, whole-grain pasta and beans,
high-calorie fish, such as salmon and tuna. Add extras to foods you already eat. Add cheese on top of casseroles, soups and eggs. Or add avocado slices, nuts or beans to salads. Don't forget eat dessert after dinner. Eat healthy desserts like yogurt and fruit pies; smoothie made with yogurt, fruit, honey and flaxseed oil with dinner or a snack.