My thoughts...lab is an energetic dog, will want to check out you and your stuff. I would use 70-200, big time!
I have two bulldogs and I use my 70-200 because if I use something shorter while in the park they come running to me for cookies, so being a tad in a distance gives me advantage. At home, different story. They are so used to me pointing that thing at them that I use my 50 f1.4 all the time few inches away from their noses because it is so light and I do not need to use flash to annoy them. Then again...they do not move much, so there is a big difference in breeds and personalities.
Consider it a challenge and have fun. I am a big dog lover and love taking pictures of my pups, cannot wait for you to post your pictures--if you can. Good luck.