How do you design a realistic magic system with logical limits?

I'm worldbuilding a new high-fantasy setting and want a hard magic system that feels balanced and logical. I want casting magic to have clear, physical consequences to prevent characters from becoming god-like.

What are some limits you've used that worked well?

I always tie magic to physical stamina. If a mage casts a massive lightning bolt, it costs the same calories as running a full marathon.