While you do strength training, you should also build up your cardio because this takes a lot longer and is harder to develop than strength. I suggest you start RIGHT NOW. You would need to include both strength and cardio training in your weekly routine. As a rule of thumb, jog 2-3 times a week (try 1 km for a start and increase distance as you progress) and some sprints too while you're at it (RL is not slow paced so you need those sprints). If you are concerned that doing this will make you lose weight, I can assure you that won't happen if you do it right. I myself is able to gain about 3kg (so far) even though I jog regularly; just that you need to have some off-days for recovery and/or strength training. Also, while you do your training, you should also watch your diet. Eat a lot of (nutritional) food and don't skip your meals. Eat big, gain big. Keep doing this, I'm sure you'll be very fit.
