We adopted our Jake (a yellow lab) from a shelter when he was 18 months old and have never regretted it.

I've seen people biking with labs. I don't know how difficult they are to train for that, though.

I would think in general that the best dog for this would be a breed that has a strong desire to please its human. I read that the best way to determine that desire in a puppy is to walk away and call the puppy and see if it immediately wants to follow you, or if it takes off in other directions. I wonder if the same test would work for a mature dog?