After a long time off (about 8 years) I have started running again and am training for New York. I went from 0 to 35 miles/ wk in 3 months and my longer runs are 1/2 marathons as of now. I have slowed down my milage increase and am keeping it at 10% at this point. I have always been thin - 5'7" and 125 lbs and I am currently experiencing unexplained weight gain since I started running - as in I can't even pull my pants over my legs. I'm not sure of the exact # on the scale as I'm afraid to look - but I don't understand why this is happening and it's reallly frusturating. I'm sure I am eating a bit more because my body is craving more calories, but with the amount I am running - I can't see how this could really be an issue. I did a 1/2 last Sunday and this week has been the worst so far - clothes that fit a week ago I can barely get on. It also seems as though I am significantly heavier at nighttime. Has anyone experienced this? If so - do you have any tips or info for me? I am finding this very disturbing. I am a 27 y/o female. Thanks.

I'm not exactly sure about this but 5'7'' and 125 pounds is not exactly thin, more like a but less than avergae. My sister is 5'6'' and weighs only 107 pounds. Um.. don't think there is any way to you can gain more weight. I am right now gaining about 1.2 pounds a week with training program I have now mainly working on muscles but I doubt thats the case. Maybe it's not the quantity of food you're eating but the type of food?

I too am training for first marathon--I am 27 yr old female and experiencing same thing...although I wish I could say I was your height and weight, I am 5'3" and currently 130 the scale just keeps going up and my pants arent fitting even my fat pants I cant get them up or they are suctioned onto my legs...I dont have an answer but am looking for one..I seem to get bigger at the end of the day and I am not seeing any increase in tone , my legs and butt just seem to keep getting larger...I am running half marathon distances or more now once a week then 6-8 miles for my alternate runs...ANYONE with tips advice please this IS very discouraging trying to accomplish a marathon but getting larger at the same time?

I think 5'7" and 125 is thin (I am 5'5", same weight), but depends on body fat %. Anyway, I found that my metabolism changed through my 20s (I am 30yo now) and I've had to change my eating habits and workout habits to maintain muscle mass and keep off fat. Distance running is massively catabolic on my muscles and I tend to get softer when I do running alone. I found that if I did my heavy lifting/weights, did my anerobic activities (I am training for amateur boxing), drank lots of water (>2litre per day), and ate a 40-40-20 percent protein-carbohydrate-fat diet at least 4x per day, I gained muscle and lost fat. It sounds like your body is adding muscle but not losing fat as quickly (keeping total body volume/clothes fit the same) and/or you aren't drinking enough water so your body is holding onto fluids. I have a busy work schedule and a travel schedule that is disruptive, so my training suffers at times. I try to compact my time allocated by making my weight training HIIT (high intensity interval training), the weekday running into shorter length interval runs or hills first thing in the morning, and do the longer runs on the weekend. So 1.5hrs activity/day Mon-Fri (unfortunately can't spend much more time than that), longer on the weekends. The food part is harder, I don't have much time to cook so I have 3 places where I get the same food all week long. Sorry, it's a lot to ask for and boring food-wise but it works for me.

You could be experiencing water retention, regardless of how long you have been training, sometimes unexplained gains are due to unbalanced electrolytes and food combo. get your self a bio impediance analysis see what that tells you other than that try to train in your aerobic range, as difficult as that maybe.