It all comes down to Discipline! How bad do you want it! For the most part less weight means faster times. Like Steve said depending at what distance. Alan Webb in the mile can run sub 4 at 145. But most of it is pure muscle. When training for longer distances especially Marathon and up, Weight is more time consuming. Think about carrying two 10 lbs weights with you and go run a twenty miler , then do it without the weights you’ll get the drift.
With a house of 4 kids , we have so many Twinkies , HO-HO’s , Cookies, fast food it’s scary. If it’s in the house you will usually find a way to eat it. I do a few weird things to help me. I drink Hot Tea , when I get the mid day urge. A lot of times if you drink water or some beverage it will curb the appetite. I also eat three meals a day , no snacking, and when I eat I eat a lot so I’m not hungry in the middle. Now the last one is different then most. I try to eat late say 8-9 PM. For me I will eat something good , like I rotate, Pasta, chicken , fish , steak with Veggies through out the week. When I do this I end up eating a good balanced meal, instead of eating at 6, then being hungry at 9 , and grabbing something high in fat or calories that I don’t really need. So I’m always looking forward to dinner and a good meal. That’s different then most but it works for me. The other thing I do is give myself one day where I can eat whatever I want , such as Reeses white chocolate peanut butter cups, ice cream , whatever. I find that I don’t abuse it anyway and it still makes it easier then totally giving everything up. My best weight for racing is 148-150. But I usually hang around 150-155. depending on how many miles I’m running. If I did not run but ate the same I would weigh around 170.