I have also used myfitnesspal, but only for brief periods of time. I'd get diligent and enter in recipes and then gradually the diligence would fade. If you choose to track calories, it's a pretty good app, though only really "convenient" if you eat prepackaged processed foods. I pretty much always eat real food, so you have to estimate portions and stuff... I tended to get obsessive about it, trying to "bank" calories for the end of the day so I wouldn't "run out."
At 5'7", I ran for 15 years with my weight in the 137-147 lbs range, eating the average American diet, and also a couple of years (not consecutive) vegetarian. Last year I went low-carb high-fat. My weight has ranged from 124-127 lbs over this period of time and I've PR'd at every race distance I run over the past year. I think everyone just has to find their sweet spot... a combination of calories in-exercise out that puts them at a weight where they feel strong and energetic. As runners we all accept the concept of different paces for different people... it's not hard to expand that out to different height/weight ratios for different people. And just as we try to get our paces where we'd like them to be, the same is true for all the other stuff.
My attempts to get taller have so far failed.