I'm also pretty obsessed with having a trim ab/wasteline so I'll give my 2 cents.
I've played around with ALOT of different exercises and regimens. First and foremost, if you are that skinny I don't really see why you need to run so much in the first place. It is important to keep the fat off the top of your muscle, but weighing as much as you do I don't think that's much of a problem. Take care to eat healthy as well and take in a good portion of protein. You can't build muscle if you don't have the building blocks to support it right?
Besides keeping the fat off and eating right, you will need to obviously strengthen your abdominal muscles. I said before that I tinkered with lots of different exercises. The most effective ones that I've found would be the following:
(note that these are most-definitely NOT the right names, but you should get the point)
Sit-up: This one is obvious, though it is also controversial since it may not be as effective as others. If done improperly, it may also damage your back. Despite all this, I find sit-ups to be very beneficial to my upper abdominals and obliques. If possible, try to do them on some sort of padded machine. My college gym has a few of these cool things that you can lie on and do "inclined" sit ups without hurting your back. I'm not sure how to describe this thing, but cheers to you if you can find one. Repeat according to your own strength.
Legs in the air crunch: Lie flat on your back and raise your legs with knees bent at a 90 degree angle (pretend like your legs below the knee are resting on a stool or ball). Do a normal crunch with legs remaining in the air. You can do this with your hands behind your head, or if you want to get more out of it you can do it with your arms simply in the air in front of you. Repeat according to your strength.
Legs in the air crunch - BICYCLE STYLE: This one is probably the most effective one of all of them. If you could only choose one to do, it would be this one. Similar to the one before, get into the same position with legs in the air and knees bent at a 90 degree angle. The only difference this time is that when you come up,
have your right elbow touch (or try to touch)
your left knee. Go down, then come up again and
have your left elbow touch your
right knee. Keep alternating as you come up. Repeat according to your strength.
Now, these exercises mostly concentrate on your upper abs. For awhile these were all I did, and I was worried about why my lower stomach wasn't looking as good as the upper, which is why I'll now introduce exercises for the
lower stomach.
Leg raise - Lie flat on your back, and put your hands under your buttocks. Using your lower abdominals, lift your legs up to the point that your body is making an "L" shape. Hold for a few seconds, then come down slowly. Repeat according to your strength.
Kicking leg raise - Same as before: lie flat on your back and put your hands under your buttocks. This time; however, lift your legs about 6-12 inches off the ground. Keep them there. At this point, lift left leg 6-12 inches above the right. Then bring it back down to the same level as the right (keep them in the air though). With both legs still in the air, raise the right leg another 6-12 inches above the other left this time. Repeat this motions as if your kicking the backstroke in a swimming pool. Make sure to use your abdominals to lift your legs, of you're just wasting your time. How can you tell if you're using your abdominals? if they are burning, then you're using them lol. Repeat according to your strength.
V-leg raise - Another similar one, but a bit more intense: you know the drill - lie flat on your back with hands under your buttocks. This time, spread your legs apart like a V, kinda like when you go up on a jumping jack. Then; same as before, raise your legs for a few seconds then come down slowly. Repeat according to your strength.
It's important to know that for all of these
lower abdominal exercises, don't let your legs touch the ground until you are done and ready to move onto the next exercise. Otherwise, you wont feel the burn and it will defeat the purpose of doing it in the first place.
I hope this helps. Just a disclaimer, these are most of the things I've found to be effective for myself. I could be right, I could be wrong. If someone wants to correct me or add, please do.
Good luck.