Any exercise science types or simply avid bikers reading this?

I've been biking a lot recently. I already noticed (and mentioned on this blog) last year when I took out my bike a few times that compared to running, biking is a lot more about muscular endurance and a lot less about cardio-vascular endurance. At least for me it is. Is this how it should be? Am I not getting as good a workout because my heart and lungs aren't feeling as challenged? Though of course muscular endurance is an important part of overall fitness too. Does it depend on the way I bike? I tend to shift to the highest gear that's comfortable--would I get a better overall workout if I left it a gear or two lower and pedaled faster?

My goal, as with running, is not to get into competitive shape but simply to keep healthy, if that has any bearing on the answers. Though, I have toyed with the idea of doing some 5k runs...I just haven't gotten around to signing up for any. I have absolutely no idea what I'd be able to run today. I only ran a 5k once, and that was in high school--I think it took me about 20 or 21 minutes. There was a time in college when we were running about an 18 minute 5k pace for training runs that were twice as long, so I'm sure I could have been around 15-16 minutes given a couple races to get used to it. But today all my running is with a jogging stroller, and I don't bother paying any attention to my pace.

There is a mini triathalon some friends of mine did here in Ft. Collins this summer--that would be fun too, though the swimming would probably wipe me out. I used to be a lifeguard (for 9 years!), but I was never great at the swimming endurance tests--I always passed them, but they would leave me dead tired. I'd probably just breast stroke the whole way--slower, but I'd come out and still be able to bike and run.

Oh, one more question: anyone know how to compare mileage biking to mileage running. My workouts for running are usually 4.5-6 miles, depending on the route. I've been biking anywhere from about 8-16 miles, depending on which park we bike to.