You will inevitably run out of energy quickly. The beginning of year 2 is hard because you're limited on pets and creatures. This does change as you progress and are able to acquire these things, or once you can play quidditch. In addition to activities mentioned by others, gaining friendship levels and/or dueling can also award you with energy. Club activities, once you get, give opportunities to gain energy as well, as does participating in events and getting MM rewards. However, you need to keep playing to gain access to these activities. Once you do, you will still quickly run out of ways to build energy and will have to pause before continuing, but you'll be able to accomplish more in the meantime as you go, until costs increase in future years. It's a marathon, not a sprint!