I've haven't taken annie on a long walk in awhile as we have been busy... But when I was training for a 5k (3.5 mile) she would go on jogs with me with no problem. She is about 5-6 lbs and at that time she was probably 10-12 months. But even from maybe 4-6 months on she would go on very long walks. We live in the NYC area so walking is what we do...
She has a blast... And when she was too little, she would let us know when she was tired... She would just plop her littly hiney down and wouldn't move...
So I'm sure you'll know when you're little one has had enough! You'd be surprised, what they can take... When we would get home from our runs, I'd be dieing and she'd be like are you kidding we're done?
