Basically, if you're trying to create a metaphor, you're looking for two things that are basically not alike, but potentially have some sort of shared connection or similarity in a reader's mind if presented together. Similes compare things using "like" or "as", but metaphors are tricker sometimes because one is basically stating that one thing is another. Some examples of metaphors one might see:
The trees whispered softly as the storm clouds marched across the sky.
These two examples are actually examples of a special type of metaphor called "personification" because one is comparing an inanimate object to a person by giving it human characteristics. Trees don't whisper, and storm clouds don't march, but people do, and this figurative language perhaps creates a better picture in a reader's mind than simply saying "There was a breeze as storm clouds moved in."
My college roommate's car is a moving landfill.
Clearly, the car is not a landfill, nor is the landfill a car. However, by stating that the car is a moving landfill, one conjures up images of a vehicle strewn with all manner of garbage, perhaps fast food wrappers, soda cans, bank receipts, newspapers, etc.
If trying to create a metaphor for a muscular build, start first by listing as many things as you can think of that suggest strength and health; from there you might be able to determine an appropriate metaphor to assist you with your description of the word "muscular".