Lari
As a GP and an ex-smoker (all of 3 weeks so far - the smoking, not the doctoring...), I agree with everyone above. Although most ex-smokers find their lung function takes a long time to return to normal (around 6 months on average), the usual complaint is "quitters cough", which isn't really what you're describing. But there's no denying that fags interfere with the normal mucus production in the airways, so when you stop, there's a bit of "catch-up" mucus production.
Do you get breathless at any other times? What about the walking? Night time? Early morning? Do you have a cough? Because without any of these, I'd be fairly reluctant to label someone as having asthma, and, in all honesty, would also be fairly reluctant to investigate further.
If it's just on running, I'd say keep going - you'll find the lung function will improve with time (as I said, about 6 months on average). And in answer to Slo'Boy, not everyone is as lucky as you... as for me - it's my first post-quit run this evening, so fingers crossed...