Most people who get lung cancer have smoked upwards of 50 years, averaging over a pack a day. Let's see, that's 20 cigs x 7 days x 52 weeks x 50 years....hmmm, that's about 364000 cigarettes. Right, forgot to factor in leap years. Okay, that's 364240 cigarettes. Yes I'm aware some people get lung cancer after only just 20 years of smoking, but this is EXTREMELY rare. And you know what? They've still probably smoked over 100000 cigarettes. A 20 year smoker only has about a 2% chance of getting lung cancer, and you only smoked 1? Mathematically, your chances are 0.00002%, or about 1 in 5 million. Actually, the official stats aren't linear, so you're chances are probably closer to 1 in 500 million.
You're fine kid.
From what I understand lung cancer is not hereditary. Also, was your father exposed regularly to asbestos or benzene?