I've been cycling since August. A bit over 600 miles later, I can definitely ride faster and longer than before.

However, is this because I'm getting fitter, or is my body just adapting to cycling? I know I've gained a lot of muscle in my legs and butt, and my core muscles feel stronger which is a plus of it's own, but how do I know how much better my cardio fitness has become?

@Joe, I do ride much farther and much faster now. My idea of a hard ride early on (I only ride at lunch, so they're all short) was like 7 miles at maybe 12Mph. Last Friday I did 13.2 miles at 14.8Mph, all over the same terrain. So I'm a better cyclist, but how do I separate body changes and adaptations (more leg muscle, muscle adaptation to pedaling) from the cardio fitness (stronger heart operation, better lung capacity, etc)?

I do ride with a HRM, but those numbers don't seem to change much.