The Holyrood 9a is a great spot for a pre session lunch (or a mid session food binge) - not far along from BrewDog, a great menu full of interesting and delicious burgers and around 20 taps (both cask and keg) - excellent service etc every time I've been.
Plus, if you're looking to try a real cask ale haven, then Cloisters bar (ten mins further along from Hanging Bat) always has the best part of ten tapped - not always a groundbreaking selection but in amazing condition and in a really cosy old boozer.
Edinburgh is one hell of a city - enjoy it!