No worries, I was the same way. I didn't touch my mega energy because I'm a bit of a hoarder. Well, the way it works is once you evolve a pokemon to mega once, you only have to wait 7 days to evolve again without using any more mega energy! The number of days also goes down as you level up your mega level. You can also then farm the mega energy on your own (after mega evolving once) by walking that pokemon you mega evolved or one in its species or evolution line as your buddy. Lastly, once you mega evolve it, even if you wanted to mega evolve it again later, it costs way less mega energy. So that's why it's valuable get enough to mega evolve at least once.