You can add concentrated calories in the form of healthy fats, such as nuts and oils. One extra roast beef sandwich with mayonnaise, a glass of 1- percent milk and an apple is close to 600 calories. You do need adequate protein intake, about 70-90 grams a day. With the carbohydrate and fat calories to support the precious protein, you can put most of this weight on in your muscles. Of course, you need some strength training to promote this.
Aerobic exercise is great, but does burn calories readily.
You did not describe your activity level, so if you are increasing exercise from what you do now, you will have to add even more calories. Adult active men need between 2,000 and 4,000 calories a day. You may see products advertised to help you gain weight. You first should determine if you could add real food to your diet to gain weight, and then supplement with these costly products. A registered dietitian can design a weight-gain program specifically for you.