We walk 1.5 miles in the morning, go out to throw the ball a couple of times a day, then gets another mile walk at night. He is crate trained, and I admit, he is bored throughout day, but we're home with him (I'm SAHM) and he's only crated at night and when I leave the house.
We have a 3 1/2 year old lab puppy who is finally outgrowing the chewing. It's wonderful that you are taking your pup on long walks, but we always refer to walks as "reading the newspaper" for labs. They like to go out and sniff and check out which other dogs have been by since the last time they visited, but it is not really exercise for them. Our lab runs beside DH when he bikes, or he will run around the block with the kids. He will fetch until he loses interest, then we will chase him around to keep him (and us!) moving. Labs have boundless energy. Another idea is to look into some playtime for him at a boarding place. The place where we board our dog has two long play sessions each day, both for boarders and for day "campers". Our dog will sleep for days after he has been boarded.
If you have a power chewer, I agree with the PPs who said not to give a towel or rope toy. Our pup loves to play tag with a rope toy, but if he is left unattended, he will shred it quickly.
We like the JW toys (available at Petco and probably Petsmart) - he has several that he has not destroyed. Kongs are also good, although our dog did eat one.

One of the Kongs (maybe the blue one?) is visible on an x-ray, just in case (don't ask how we know that).
The white Nylabones have held up well - better than the off-white ones.
If your pup is chewing his bedding, you might check out a Kuranda bed. They are advertised to be chew-proof, and our dog has not destroyed his yet!