Hi - in the past few months I've started training for my first full marathon (have done a few halves and tris in the past). Suddenly with the longer distance runs my weight is just falling off me and not in a good way. I've gone from looking strong and healthy to skinny and don't feel as good. I'm taking gels on my runs and trying to eat healthily as usual but it keeps dropping. I'm healthy otherwise - any advice from seasoned marathon runners please?