I have a similar commute length and generally have been fine with just keeping stuff in tupperware, and putting it in my panniers. However, I don't do frozen meals and usually just have dinner leftovers. Never had to worry about the stuff spoiling, even if they've been sitting in my bag for an hour before I get to the fridge.

Also, like many other posters, I keep a small pantry in one of my desk drawers with stuff that doesn't require refrigeration and can keep for a moderate amount of time (loaf of bread, jar of peanut butter, bag of nuts, block of cheddar, bag of dried fruit, rice crackers, etc.) Most of that can be grazed through for a light lunch.