Assign and update
As you're creating your document, instead of bolding, assign one of the heading styles (don't worry if you don't like it - you can change that). Once you've assigned a style to a piece of text, highlight the text and make the changes until you're happy with the result. Then you can update the heading (see image below) and all instances throughout the document will be updated!
You can even set the defaults for all future documents you create by selecting Save as my default styles from the Options at the bottom of the styles list.
Table of Contents from your styles
Once you've formatted all your text using styles, you can quickly create a Table of Contents. Just select Table of Contents from the Insert menu. Google will automatically insert the text from your styles and indent the subheadings like this:
Check out the sample document
Check out the sample document