You don't really need to incorporate intervals into your training, just climb every hill in zone 4 or 5 (or at a 7 or 8 out of 10 on a scale of perceived exertion) and coast down to recover. If you ride hilly routes (which is a good idea) the intervals will come automatically and your cruising speed will increase as a result.

The idea is to be able to put out more power for a given heart rate. That way that pace where you feel like you can keep it up forever gets faster.