Does your dog want to wee or poo every 10 seconds too? It is indeed completely normal for dogs to want to leave their scent, especially where they smell another dog's scent.
I noticed that a walk in a novel area often 'slows me down' even more as Mr / Mrs dog wants to leave more marks.
If your dog really needs the exercise it could pay off to stick to a routine walk as your dog may 'go' less.
Too many marks in a row can sometimes cause runny poos and may prompt (light) diarrhoea. As a pet sitter it's important to keep an eye on this as you are most likely taking different routes than the dog's owner.
Spending time at a dog park will encourage your dog to play, chase, jump and yes, mark again. This type of exercise and social contact is a great way to complement regular walks.