
Dear Finn,
Your breeder is absolutely right! Well done on finding a good one! If a dog like this is given loads of gallopping exercise it can lead to bad joints and other troubles in the future. Wait till her skeleton is fully grown before trying to build up muscle. Equally, don't let her get fat! At the moment she should be VERY slightly roly-poly - this will disappear as she grows, and is a useful "safety-net" if she picks up a mild tummy upset. Monitor her weight (best done by feeling her ribs - if they're buried in fat, she needs to lose some, but this is done with a pup by keeping their weight steady until they grow into it)
If you let her play in the house and garden, and when she's completed her course of injections, take her out for training/socialation walks, this will be fine. Lots of short walks is better than one or two long ones at this stage.
In the meantime, carry her around town so that she gets to see crowds, cars, lorries etc from the safety of your arms, where she won't get diseases and won't be scared.
Don't worry, the long walks will come when she's grown up - you should have a good 14 or so years with her, so don't rush it!
Good luck.