Definitely a topic I'm interested in, since it looks like I am developing Cushings Disease :) You can find some studies on pubmed which indicate that overtraining can lead to chronic elevated cortisol. I'm not into cycling, but I'd imagine 200mi a week takes quite a bit of time?
I usually do 45 minutes of cardio a day 5 days a week, pilates 3 days a week, and light weight lifting 3 days a week. I have 24 hour urinary free cortisol tests frequently, and my results have not varied whether or not I exercised that day. And despite my cortisol elevations, my doctors have encouraged me to exercise more. This will help me combat the weight gain and bone loss that come along with my condition. I definitely believe that despite the temporary increase in cortisol, most people are doing their body a lot more good by exercising (which lowers your stress level and has a beneficial effect on cortisol in that respect).
Warning signs to watch out for: easy bruising, disturbed sleep, abdominal weight gain, a rounding of the face, fatigue, and mood changes.
If you are concerned, I would ask for a 24 hour UFC test. Serum cortisol tells you next to nothing.