obsessed with squirrels

Bongo, most domesticated dogs are 'hard wired' (genetically predisposed) to some form of this predatory type behavior.  Especially with a working breed like the the Lab.  They just can't help themselves.  I use to run with a Golden Retriever and German Shepherd (together) and I had very good luck with using a Halti brand head halter.  It gives you more control over the dog and the dog is more submissive and listens better.  When we would encounter a squirrel, I would just say in a big Alpha voice, "leave it!"  They were quickly conditioned not to bother to challenge the Halti which tightens up if they pull away (with out you having to correct the dog) or me. 

When you are not running, playing lots of ball will help get out some of that retrieving instinct and energy directed at small moving objects.  It  usually helps to desensitize a dog to an object or animal but that is hard to do when it (squirrels)  runs wild.  Good luck!