Exactly as ondor says.
I have a pedigree black Labrador who is from a working line. Far more slender than the show bred Labradors. He is now 18months old and just about fully grown (minus a little filling out) and now hovers between 28-30kg (show bred labs will generally be closer to 40kg!)
He is more than happy alongside the bike. I’ve only started taking him out in the last few months but he will gladly do 5miles or so without too much bother and has no problem keeping up (or waiting for me on climbs).
I always take some water with me and a few treats as rewards when he’s behaving.
Wouldn’t change him for anything. His temperament is spot on too, absolutely besotted with our nearly 2year old daughter and the bond they have is incredible. Perfect family companion!
This is Jinx
And this was in the summer (him 1year, daughter 18months)