When training for my half, I was also running my long runs at my half marathon pace, I didnt' know to run them slower. Now I'm also going to start training for the Edinburgh Marathon (my first one too!)
Was recently told by a running coach that the long slow runs teach your body to use fat stores for fuel during the marathon, leaving the carbohydrates for when you need them - the 2nd half of the race (6.2 miles ) Your body will become more efficient by training this way, so that when you come to do the marathon, it will know to use the fat first. She also said what has been mentioned above - when you get to the part of your training plan that has your long runs at half mara or greater, you will only end up tiring yourself for the training the following week, requiring more recovery time.
I also get too hot when I layer up in the cold - legs are ok in tights, but I find my top half overheats, and wanting to strip down to sports bra even in the cold (I don't, as there are kids and old people using the path I run )
I recently read on here about using a base layer, so I might give that a go - one with long sleeves to cover my hands too (not usually a problem, being a shortie!)