Hello there, MUHV!

From my own experience playing other games, I understand where you're coming from.

However, I wish to offer you my advice, which would be using your Energy wisely.

First of all, the easy way:

You can get Energy from linking your Amazon Prime account to the game. You'll be able to get 700 Energy from it.

And finally, the normal way:

1). You can get Energy from completing Campaign missions (from start to finish only, no re-challenges). 

2). Following and completing quests, challenges. 

Doing these two things will allow you to grind up more Energy as they have rewards which help.

3). After you reach a certain level, you will unlock Events which provide you with goals that when completed, reward you with Energy, which is what you need.

Amazon Prime will only provide you with just one stack of 700 Energy (to the best of my belief), while I did take this advantage, I used up all of it grinding items in Dungeons.

As long as you follow missions, you should be able to spend more than 15 minutes using your Energy.

This game isn't heavily focused on P2W, so don't worry about that. You can also get rewards for joining a Clan and participating in Clan Boss Fights after you reach a certain level. 

If there's any other questions, I'll do my best to answer you, however note that I, myself, am a novice. ^_^

P.S Forgot to note, that Events aren't limited to Energy, but also with different rewards as well.

Thanks, hope this will help!

Best Wishes on your Journey!

Richa