2.5 miles of leashed walking for a 14 week old is a lot of forced, repetitive motion in my opinion. Labs are a breed prone to dysplasia. Even if the parents have had their hips/elbows graded by OFA as passing (fair, good, or excellent), too much, too soon with a growing puppy can negatively affect their orthopedics. I think plenty of well-meaning pet owners are the cause of their dogs’ joint problems. Maybe read up on appropriate exercise for medium-large breed dogs. Labs’ growth plates are still open often until 18 months. If you’ve neutered/spayed early, it can also affect joint health. Here’s one resource regarding puppy exercise. https://www.puppyculture.com/new-exercise-chart.html There are plenty of things to read online.
I allow my dogs plenty of free play as puppies but leashed walking at 14 weeks is short and sweet and more about reinforcing good walking manners. An intense mental workout as you ask your young puppy to heel/follow/turn/sit for 100 yards is just as exhausting to them as walking for miles and miles, if your concern is tiring a puppy out. At 14 weeks my puppies are getting short, intense training sessions, short walks, plenty of free play in the yard with me. Definitely no long walks, no jumping for frisbees, and I don’t neuter until around 2 (dogs I’m showing are not neutered at all).