If your trying to gain muscle mass then your not going to be able to a ripped 6 pack, however you should be able to maintain around 10% body fat (which means you'll still have abs). You should eat a minimum of 16 calories per lbs lean body mass, which is probably around 2500 for you. If I were you, I would try to eat closer to 3000.
Next, you need to lift at least 3 times a week, 4 would be better for faster results. On your none lifting days, sprint (on flat ground or a hill) or push a weighted sled. This will help keep the body fat off and help build up the muscle in your legs unlike traditional 'cardio.'
Finally, make sure you get your pre and post workout down. Some protein before working out and protein and carbs afterwards for maximal recovery and protein synthesis.