Hi John, what makes you feel that you have little lung capacity? I see from your profile that you've been running over 10years. If you've just started swimming, then you're probably having problems because the muscles needed for swimming are untrained, and as a result your aerobic system has to work overtime to cope. Such is the specificity of training i'm afraid! Just out of interest, how's the running? What's your race times like? I'm sure its nothing to do with the aerobic system at all and it stems simply from the fact that, as said, your muscles are untrained for swimming.
Jonny