Your lungs will definitely recover. You really have no idea how much cigarratte addicts smoke. Your once a week joint is absolutely nothing compared to them. If they could do it, you can do it. And it's not like it's a chemical that messes up your lungs when it comes to weed (like nicotine, with tobacco), it's the smoke itself. It's not too dangerous unless you do it every day for a long time. And it's still not as bad as cigarrettes. It's true the weed is equivalent to 4X a cigarrette - but think about this: the average joint weighs/contains less than the average cigarrette. And cigarrette smokers likely to get cancer smoke a pack a day...most regular weed smokers do a joint or two a week. Do the math, no where near as dangerous. Even if you do a joint everyday (which is pretty stupid, since it's not even addictive), it's still healthier than cigarrettes.
So yeah, with time, your lungs'll definitely be better.
P.S. I'm not a regular smoker (I smoke on certain times, like celebrations or something), but I've researched a lot on this subject. Weed is pretty harmless, unless you're the type of person that is easily obsessed - with anything, whether it be food, video games, etc.