Most people think they're getting in better shape, the longer they're able to do cardio. We've all heard those friends and family members that say, "I used to get tired from a 10 minute run. Now I can run for 35 minutes straight without getting tired!"
Doing cardio for extended periods of time is actually counter productive to you burning fat. You see, our bodies don't like our heart rate to rise too high for too long. For example, when you're in a car and you step on the gas, the RPM's increase rapidly. Now if you don't take your foot off the gas, the RPM's will max out at 5,000 and then bounce back down after hitting the rev limiter.
If the car didn't have a built in defense mechanism(rev limiter), the RPM's would have continued to increase until the engine blew up. This is a similar process to how your body responds when you elevate your heart rate for too long. The body simply conditions the heart.
When your heart becomes more conditioned, it actually burns less calories per cardio session. Which means less fat burning. And worse yet, your body is more susceptible to burning muscle. This is why I advise my clients to control their cardio intensity and duration.