Cross-training (specifically weight-lifting) might be helpful to gain musle and it is also beneficial in assisting running.  Of course, you may want to boost your diet with higher-calorie/nutrient rich snacks throughout the day. (trail mix, avocados, dried fruit, nuts).  Consider talking to a dietician or nutritional expert about meal planning and/or any appropriate suppliments.  Also, it may be a good idea to get a physical to check everything out to make sure nothing else is going on or interferring with your metabolic process. 

Wishing you the best

Good advice on both above posts. It does sound like you need to increase caloric intake. If your goal is to add lean body mass you need to add resistance training. Your post workout recovery meal could be an issue. Within an hour of exercise cessation eat a meal that has a 4-1 carb to protein ratio with very lttle fat.(example; 25 grams protein, 100g carbs, 5g fat). The carbs will be used to replenish muscle glycogen and the protein for building blocks. Too much fat will slow absorbtion at a time you want it to be fast.

Overall I recommend a diet of 30% protein, 30% fat and 40% carbohydrate. Works well for my clients.

Personal Trainer Los Angeles - Studio City