Sprint. Check out High interval training. Sprint, Jog, Sprint Jog. Sprinting is the only thing that your body doesn't get used to. It get's used to jogging, so if you jog a mile every day, your body gets used to it and doesn't burn many calories. Changing the pace of your running is the best option. Get to a gym if you can afford. I told them I'm wanting stamina, I forgot the name, but you're running isn't constant. 1 and half minute of say 8kmph, then 1 and half minute of 3% 'hill' at 10kmph and so on. This stops my body getting used to it, and when I can do this workout easily, I'll get it changed and push faster and longer. Sit-ups don't do much by themselves. You need cardio. Go on a bike ride and again change the routes/speed, go uphill and so on.
Best people to listen to are qualified people, such as gym trainers.