Do a second 1.5 mile walk each day. You need to tire them out at that age. Also, do a lot of mental games. Turn a muffin tin upside down on the floor and sprinkle the dog's food around the protruding circles, for example. There are tons of things like that.

Labs are notoriously high energy as puppies and adolescents so keep that in mind.

I have a spaniel mix that is also super high energy and we spend about an hour and a half a day walking him and our other dog.
-20 minute walk in the morning
-50 minute walk at lunch and/or we go to the dog park
-30 minutes of walks in the evening

Have you tried taking the dog to doggy daycare or the dog park every once in awhile for the dog to blow off steam? The dog park time is what makes the biggest impact on our dogs. 20 or 30 minutes at the park and they are passed out for the rest of the day.

I have a mini golden doodle. He is at home in the house alone during the school and work week. He is pretty low key and seems fine going out in the morning and in the evening for a short period. He goes outside in the fenced back yard several times when we are at home. He gets much more exercise on the weekend when he can run with a small dog who lives across the street. Our street has low traffic so he and the other dog run between our houses off leash. I am careful not leave him at home alone for more than 8 hours because he enjoys human contact.

This sounds about what we did with our lab.

Probably half an hour to an hour. I have a small, easy going dog though. Big dogs like labs do need more stimulation and attention.

Not just in each other's company...but walking, playing, training, take to the park, petting, etc. (good one-on-one time)? Just curious how much time/lovin' other people give their dogs. My dog always wants me to play MORE with him (naturally), but is this sufficient/normal? I give him one 1.5 mile walk a day, throw toys for him to chase in our (big) backyard, one or two rub downs, a couple sessions of tug, petting and sweet-talk here & there as I pass him. I rarely take him to the park. Kids give him attention after school but don't really "play" with him. He is an 11 month lab.