I'd say warm up and get going on a cardio piece, preferably not a bike at first. Those are too easy, and too easy to cheat yourself.
About 8-10 minutes of getting lose and get a sweat going. Then 3-5 minutes of BUSTING ASS. Absolutely excruciating effort- get your mind in the right place and get the blood pumping throughout you're body, fast.
Now move to the circuit training you have planned. You're already warm, dive right in, stretch before cardio if that's an issue. You burn that extra glycogen out of your muscles by doing a bit of cardio first, then following weight resistance.
You got that pesky stored glycogen the hell out of your muscles now. This is the time for your cardio. Your tired. You already did cardio. But this is a great way to burn fat more efficiently. You want this main cardio, to be a high-moderate to intense. This one you should be thinking "holy fuck" on your 15th minute. And you should be cringing to think you STILL have 5 more minutes when your 25th minute rolls by.
30 minutes of intense cardio is better then 1 hour of light to moderate, any day.