We can walk more if the dog needed it so good to know that we'd need to plan for more time. I think that would be the ideal time from our perspective but we'd have to adapt if it was required. I just wouldn't want a small dog that got puffed out too easily.
The vast majority of small dogs are more than capable of running for hours. In fact, it's the giant breeds that can't often cope with lots of exercise due to their hearts and the strain it puts on their joints.
I would suggest having a look at the following:
Cairn Terrier - small, but hardy and can walk for miles. Generally friendly and healthy, though they do need regular trips to the groomers (probably every 6-8 weeks is best).
Shih-tzu - typical lap dogs, though fit ones can go for miles. Be careful with their face shape as they can suffer from a flat face and the associated health issues. Again, regular grooming is needed.
Border Terrier - good with people (though can have issues with other dogs), hardy and healthy - not too small either. They do need clipping/hand-stripping regularly, though.
Miniature poodle - need probably 60-90 minutes of exercise a day, don't shed (but need brushing daily and clipping regularly).