It's great that you are working on building up your endurance and speed. The burn numbers that you are getting, where are they from? Treadmill defaults basically assume you are a 6'5" super-athletic guy, which is great if you are a 6'5" super-athletic guy, but for most women closer to the 5' mark it's doubling what you are actually burning. Most people seem to agree with the fitbit math and treadmills that take your actual stats into account (I know my local Y uses treadmills you have to log into, which means that the treadmill shows burn based on your data, not some random giant guy meant to skew the numbers higher so you pick their machine) can be on par. It's about figuring out the math and if you skew average, a little below or a little above. I would say to make sure that you aren't eating those exercise calories back, but also make sure that you're not aiming for an aggressive loss per week goal. Something like half a pound to 3/4 lb per week would be where I would try, but know that it might take two months to see a definitive drop of a few pounds. As you build up your workouts, then look at a smaller deficit. Finding the balance between fueling your workouts and maintaining an appropriate deficit for your current stats and activity level can be a difficult balance to achieve.
-google first. ask questions later.