Sara,
Since your lung functions are so well, I suggest you look into what non-cfer's do to stay fit. All of this falls into the 'use it or lose it' category. As a cf'er with severe lung disease, walking is the only exercise I can tolerate. I use a treadmill on crummy days and try to walk my dogs on the nice days. Walking aerobically my benefit you, if you can do more, by all means do.
Also, staying on top of your vest treatments and drugs/ vitamnis etc and not waiting too long to see the doc when you're sick, will go a long way to keeping you in tip-top shape <img src="i/expressions/face-icon-small-happy.gif" border="0"> Good luck!
Rich