When people think of running and what it takes to be a successful runner, most of us automatically think legs. That's what I did when I started running. I've come to find out that running is much more complicated than that.
First came the realization that while my legs could go on and on, my heart and lungs couldn't. I found that when my heart rate got too high I had to walk. Breathing becomes more of an issue in the winter for me because I have exercise-induced asthma. The solution for that is to use a neck gaiter to cover my mouth as I breath in the cold air.
Now comes a second realization. My core isn't strong enough to handle the mileage build-up I'd like to achieve in the next few months. By core I mean my abdomen and lower back. I've been having a little trouble with a stiff and sore lower back for weeks now. I thought perhaps it was my mattress, which I would love to replace sometime soon. Unfortunately I am 99% certain that it's the running. Because I'm working on stamina and endurance and my back isn't strong enough to meet the challenge, I'm running into more pain than I'm used to. This isn't debilitating pain - it's more annoying than anything else. But if I'm not careful I could hurt myself.
Here are some links that show how to work on some simple core exercises that are good for runners. Remember to give each exercise time. Don't rush through them using momentum because you're not getting a good workout that way. To be cliche, feel the burn.
Bridge - Great for working glutes and hamstrings. Repeat 10-12 times.
Side plank - Or any planks really. Great for obliques, transversus abdominus (deep abs), lower back, hips, and glutes. I hate planks in general because they're hard, but I know they work. Hold this position 10-30 seconds and repeat on the other side.
Superman - Great for transversus abdominus and erector spinae (lower back). 10 reps on each side.
Take a look at the other exercises that accompany those links. They're all good at strengthening your core. Sit-ups and crunches are good, but they don't work deep muscles like the three highlighted above do. Dedicate 15-20 minutes 3 times a week and you'll start to feel a difference. These exercises won't eliminate belly fat but you'll reduce your risk of back injuries significantly. And who wants a back injury? I certainly don't!
Edit: I should have noted that I got this idea from Runner's World December issue.