Indeed, you don't need to make by hand your "Continue reading" links code, a tag that does that is provided for you: the more tag.
You can add the more tag on Edit Post by clicking on its icon on visual mode (ALT+SHIFT+T), previously placing the text cursor where you want the link to appear, or by writing the code for the more tag on the html mode. You can also costumize the message that shows up ("Continue reading"). Here's the support page with pretty explanatory info about all that: http://en.support.wordpress.com/splitting-content/more-tag/
Also, besides the more tag, there's also the next page tag that you can use to divide your post into pages. Here's the support page: http://en.support.wordpress.com/splitting-content/nextpage/