The Add-ons have three levels of difficulty: easy, medium and hard. These three levels of difficulty are progressive. So, if you complete a number of "medium" Add-ons, they'll generally be of a similar level to the first "hard" Add-on.
Here's how to change your Add-on level:
On the website
After logging in, click on the downward arrow next to your username in the top right-hand corner of your screen. Under "Settings," you can change the levels of the Add-ons under "Training" using three levels of difficulty: "easy," "medium" and "hard". Then click "Save" at the bottom of your screen.
In the app
Tap on your profile icon in the bottom right-hand corner of your screen, then tap the gear-shaped icon in the top right-hand corner of your screen to access your settings. Under "Workout" you can change your Add-on level of difficulty. Then tap "Save" in the top left-hand corner of your screen.