20 lbs of lean mass will take a while to achieve especially at a low dose of deca and test. but your heading in the right direction by starting off with low dose. your a bit young too use aas by the way. your gonna have to eat a lot, 2 grams of protein per body lbs. try about 270-300 grams a day of protein. Seems like you have a fast metabolism, i would add a lot of complex carbs i would try and hit at least around 350-500 grams a day. sweet potatoes, oats, brown rice..... you can do cardio, i wouldn't over do it if your main goal is gaining 20 lbs. Personally when i bulk i do 0 cardio, but that's just me. efa are a must also