Don’t confuse the chunky guidedog/pet/show labradors with the labrador retriever/field labrador. I think they should almost be considered two different breeds.
A chunky choc lab or any other show type will be a poor choice, they have been selectively bred for temperament, steadyness and lower exercise requirements that suits their purpose.
Perrin on the other hand comes from working and field stock, as you can see he is not chunky (29kg), has a slimmer profile along with a different shaped head/nose etc. He will happily join me for 3-4 hour rides every day of the week in the holidays and does a minimum of 2-3 hours of solid running daily (2 hours in the park in the morning with doggy pals and a 10k in the evening).
Regardless of breed, you needs to build up to the higher exercise levels and if you’re doing a lot of miles it’s worth talking to your vet & considering some food additive for joint care. I would also say that it’s worth investing in some ruffwear boots to help protect the dog’s pads if you’re doing a lot of long stuff/on trail centers/on the moor.