I forgot what your main goal was... You do not need to go for 40 minutes to tap into fat stores, but usually first 10 min you are not tapping into fat stores, just using what ever is in your bloodstream. I may have said 40 minutes, because its not too long, not too short. Wouldn't do cardio for more than an hour at a time.
If you are trying to build bulk and minimize the gain of body fat. First eat healthy and correctly, second do cardio at 60-80% of max hr for about 30-40 few days a week preferably in the AM. If you are just trying to lose body fat, High intensity is the way to go....Try different cardio machines too, and in a different order, try outdoor activities, biking, swimming, body wont become acustom to it if you keep changing it up. Im really tired right now, so if this doesnt make sense, its probably me, just let me know your questions and ill reply when im more awake.