Weighing in... blablablabla

I am not on any type of serious weight/fat loss regimen but I do have a fat loss goal.(which is not the same as a weight loss goal) I do watch what I eat- white carbs are a rare occurrence as is alcohol and anything with high sugar content. (I recently realized that I window shop the white carbs in the cafeteria at work EVERY MORNING... slightly pathetic, right?) I sometimes see lower numbers on the scale. Since I put myself on a schedule/regimen for proper hydration, I became curious about how it might affect my weight. I started weighing myself every couple of days in the morning before breakfast and coffee and everything else except going to the bathroom. Here's what I have:

Fri: 127.5 (after a work week of wise choices)
Sun: 130 (after a bbq where I ate and drank at will, but not quite gluttony)
Mon: 131.5 (see sunday)
Wed: 129
Sat: 126.75 (and I'm carb loaded for a long run!!!)

I follow a few blogs and also talk to others (in real life!) about weight loss. I still don't know how on earth people get excited about losing one pound or get discouraged about gaining a pound. My life would be a roller coaster if I did that.

What causes this tumultuous fluctuation? I haven't seen the scale go below 126, so I'm thinking that when it does (even by 1/10th of a pound) I can assume that to be a real decrease.

I do have some new abdominal definition- sometimes(that's another mystery- it's not consistent) I can see obliques pretty well, but I do not know the rhyme or reason to this. After a run I can usually see them, for obvious reasons. But sometimes I'll sit around all day, then eat Taco Bell for dinner and have great definition the next morning... I'm not obsessed with having perfect abs (or a perfect body for that matter) but I know that this is a good indicator of fat loss. The prominent fat that I have is on my hips, lower abs, and thighs. I want to shed some of it in exchange for speed and efficiency during my next marathon but I honestly cannot fathom being under 120 pounds. That isn't a super low weight for my height (I round up and say that I'm 5'3") but I imagine hanging saggy lifeless skin if I go under 120.

One thing that is not in my favor as far as fat/weight loss is that running is not an efficient exercise for me if I'm trying to reduce. My heart rate is too high, therefore I'm up in my anaerobic threshold and I'm burning more stored energy (carbs, sugars) than I am burning fat. I didn't know this until I had my metabolism tested. But when I look back at my weight loss history and I notice that I had the most success when I used other forms of cardio exercise. (elliptical, walking with a kid in a stroller, classes at the gym) So since I'm training for the Philadelphia Marathon I can't really cut down on the running, and I only have so many hours in a day to spare for fitness. Also, my basal metabolic rate is pretty low. If I want to maintain my current weight, I only need 1150 calories. If I don't exercise regularly, I inevitably put weight on. Do you know any adult human who lives in the US and only consumes 1150 calories a day? I sure don't.

I'm wondering how other people weigh in. How do you know your weight loss is real? And what do you consider your "true" weight? I just pick the lowest number because it makes me happy. I am going to try and touch 122 lbs. before November 20th. I think a 4-5 pound loss in the span of 3 months is pretty realistic.