Personally, and this is all SWAG, its sounds to me like you need a heart rate monitor, or to follow it if you do have one, and just keep your heart rate at the right level, probably around 120-135, the whole time, while sucking down your fluid of choice at a rate of about 1 liter per 40 minutes or so. I'd get one of those wristwatch sorts where you can set it to beep to different noises then set it for your low and high, which is 60 and 65% of your MHR, usually 220-age, so like for 30 it would be 114-124 and I'd set it to warn you at 115 and 130, you can pour it on or throttle back. Speeds pretty irrelevant then, and you can vary mid-work out by playing with the slope if you want to work on your stride length or push on your calves, all while staying in that range. A little juice box at around 35-40 minutes might help too, I know a lot of people have problems drinking while moving but IMO that's mostly practice, I personally like to take a few sips every 3-5 minutes, usually between songs. My own preference is an elliptical parked in front of a news channel and a MP3 player loaded with 80s music, because generally doing the 130-ish fat burn is more about boredom to a background of discomfort then the kind of near-vomit blood pounding intense cardio. Burn more fat that way, per exercise anyway, a good slow 60 minute jog will typically burn off more calories than a 20-30 minute intense cardio, but your mileage may vary, literally in this case.

But that's my advice anyway, heart monitor, and regulate your speed to your heart rate, its worked for both fat burning and endurance building for everyone else I've known who's done it too.