"Your mouth's writin' checks your body can't cash."
"If you can't take the heat, stay out of the kitchen" is a tired cliché that represents the idea of not getting involved in a situation that you cannot handle to its completion; for the most part, these situations involve high levels of stress or are physically demanding.
The key players:
In other words, when it comes to crunch-time in a tough situation, you need to be able to handle it. If you can't handle it, then you need to have backed out when you had the opportunity.