My points on labs:
1) Great running partners when trained. Labs are very smart and very trainable, so anyone with a misbehaving lab is a just bad dog owner.
2) Agree with having them run mostly on grass or dirt.
3) Agree with keeping them on the leash. Nothing pisses me off more than having to deal with an unleashed dog when I'm running. Most runners feel that way too. We don't know if the dog is friendly or not. Who cares if the dog likes it better with the leash off? He's a freakin' dog, and he'll do what you tell him to do. I'm glad you're planning to use a leash. That's what a responsible dog owner does.
4) Agree also with planning to use the dog for running for about 7 years. Age and hip dysplasia will keep many labs from running longer (hip dysplasia is a common thing for labs).
Getting a dog this summer myself. Looking forward to it.
In my opinion, for an active person, you couldn't have made a better choice than getting a lab. Awesome dogs.