You just set yourself on the express path through the DLC. Should take you less than 30 minutes to get back to the Mojave now.
To avoid it and play through the whole story don't instantly shoot everything that moves. Take a second to evaluate if it's a real threat, or not, as there is three tribal bands out there and only one will attack you on sight at the start.
I saved before I went in, so I'll just start again. The White legs attacked and killed the group I was with. I crossed to the other side and the Horse tribe started shooting and trying to hack me to death, lol. I know what to expect now at least, thanks for the info.