For loosing weight, distance is more important than speed. Distance gives time and time is what burns up the fat. You get your body reved up, but not over-reved, and hold it there. You can't do that by sprinting.
Start slow, start short, and gradually build up. GRADUALLY. Do it quickly and you hurt yourself.
Heart rate monitor is the greatest tool in my arsonal. Mine doubles as an MP3 player. So I listen to the tunes or whatever, and have the voice tell me what my heart is doing. I adjust my pace accordingly. Most of us try to go too fast too soon, and poop out early as a result.
Monotony can lead to injuries. If you plod around a flat track for an hour, you're moving the exact same way for that hour. Even if your form is good, you can do harm to yourself because of the repetition. Going cross country has you up and down hills, hoping over things and such. Breaks up the monotony and helps you avoid repetitive injuries. Though it can lead to new injuries after you trip over the curb and fall into a woodchuck hole.
There are good days, there are less good days. That's normal. Don't fight it. Some days you don't feel as good as others, some days are stink'n hot and humid, and some days you find the groove and fly. Just accept it and go with it. Tuesday I barely made 3 miles cross-country, and I had to drop to a walk twice. Thursday I did 6 miles on the same course, flying with ease.
Good clothes matter. I was darn slow learning this one. Trying to run in a cotton t-shirt and tighty-whities will destroy you. Having always done that, I had no idea I was being destroyed. When I finally bought my first set of those silly under-armor like running shorts and shirt, I was amazed. Not at the start of the run, but partway through it. Where the old t-shirt would be weighing a ton and the undies would be chaffing and bunching, I was now cool comfortable and light. Yippee! Next came those magic socks. Again, amazing. No blisters, no pools of sweat, no socks bunched up under my feet.
Shoes. Thick soles and big springy heels inspire lousy form. Look at the feet of those serious long distance runners. They are wearing minimalist shoes, for a reason. If you slam your heel into the ground, it should hurt, which would inspire you to stop that, and run right.