Hard to say exactly what she is, but she's pretty. If I had to guess I'd say she's part black lab and maybe you are right about the boarder collie cross. Either way, I'd guess she has high energy.
Boarder Collies are dogs that need a job- and not just something minor either. They need mental stimulation and can work for hours at a time intently studying. They don't get bored easily so as long as they are working, they are happy.
Labs are a bit less driven and don't need a job, but they have a lot of stamina and need to be physically worn out to be happy. A walk typically doesn't cut it either- it needs to be running.
Good luck in your decision making regarding this girl. She's already luck that you have taken her in for now. I hate reading about abandoned pets.
-----
Chelle and the rest of the crew including, but not limited to Kita and Taiko (the shiba inu wrestle maniacs), Adi (reserved and dignified tabby cat), and all 28 reptiles