So what if you want to add Heroku as a remote to a repo you already have on your machine, or maybe something went catastrophically wrong and you had to blow away a repo and now want to put your push setup back like it was.
I'll save you from having to go dig around Heroku's docs or stackoverflow. Here ya go:
heroku git:remote -a <app-name>
Where app name is what you called your app when you created it. You can find this under settings in Heroku if you're not sure or cant remember.