I am pretty fit, I was walking up some pretty steep hills with a pram and I am walking around 8kms a day or every 2nd day at the moment....it's raining so much!!! I wasn't out of breath but on my way there, (a conversation may have had a few pauses!). When I did stop I drank water and waited till I got my breath back.
I was just concerned that there was a limit that I should be aware of.