How far is too far for a dog walk?

[Sargeant Orange]

My 6 year old lab often comes with me when I take the mountain bike out but recently I've been upping my mileage & I'm a bit concerned he might end up with some long term damage (joints etc) or just exhaustion. He is pretty slim for a lab (27kgs).

We've just come back from an 18 mile ride, averaging only 6mph as I tend to slow right down on tarmac parts to save his pads burning. He seems absolutely fine & is currently throwing his ball at me to play again.

Any advice or experiences?

[Sargeant Orange]

With the dark nights it's only once a week currently, probably twice a week in summer. The rest of the week consists of 30 minute ball chasing sessions every other day.

I guess the secondary question would be what age should he be winding down the vigourus exercise.