Actors go through gruelling workouts to get prepared for blockbuster movie roles. While you may not need to bulk up for a fight with Thanos, you can follow along with the type of routine Benedict Cumberbatch used to become the Sorcerer Supreme.
Fitness and comic book fan site Superhero Jacked explains how Cumberbatch got ready for his role as Stephen Strange. While the role of a sorcerer doesn't require a ton of visible bulk, the actor went through an endurance routine that would get him prepared for multiple days of filming long fight scenes. He also did quite a lot of eating.
Based on this information, the site put together a similar (if less gruelling) routine that you can follow. For diet, the site recommends trying intermittent fasting.
They also have a detailed workout routine that incorporates both bulking and endurance training. Obviously, you may have to adapt your workout to your body's needs and what you want -- and keep in mind that actor workouts are often very extreme to meet shooting schedules -- but it's a fascinating look into the ways that superheroes get themselves in shape before saving the universe.