You don't want to push them physically until they are done growing
But with small breeds (under 20 lbs full grown) their growth rate is really quick - they are skelatally done growing at 8 months, and by 10 months they have developed adult muscle as well.
My little dogs are 'professional dog walkers' with my boyfriend - they go on two intense 1 hour walks with a group of 7-8 other dogs every day.
Now you have to start slow with them to make sure that you allow them to build muscle and cardio conditioning that they're comfortable doing the distance, but they're totally capable!
I would say that my little dogs ( 5.5lbs and 7lbs) probably do about 4-7km a day, and they're off leash, so they're covering more distance than even that, and they're happy, healthy, balanced and content!
Meghan