I'm an avid runner (currently training for a 1/2 marathon and soon a marathon) and i have just started my fitness plan through this website. My question is...should i be using protien powder and what would the benefit to me be? The only protein i get during the day is in my peanut butter in the AM and then some sort of protein for dinner, so probably not enough. I'm struggling to loose the last 5 pounds and am worried that the protein powder will puff me up.
Protein powder alone won't make you "puff-up". The only way you will gain weight is if you are taking in more calories than your body needs.
You need to determine how many calories your body needs per day to maintain your weight at your current weight/height. You can find a decent nutrition calculator on google to figure it out.
From there drop 200-250 calories per day and see how that takes you.
As far as the protein powder goes I think you have a need for it since you definitely don't get enough protein per day. As long as you use it and stay under the total calories per day you need then you won't puff up.