I have two shih tzus aged 7 and 8. My boy loves walking and thoroughly enjoys all walks whether its to the corner shop or a ramble in the forest. My girl however will walk enthusastically for about 5 minutes and then decides she's had enough, lays down and refuses to go any further. I have been known to carry her the rest of the way (strange looks/comment from other dog walkers) or else just try and coax her slowly and cut the walk short.
She is blind (had both her eyes removed a few years back due to ulcers) but I honestly don't think that's the problem because she rarely bumps into things and has never hurt herself because of her "disability". She's great and because of her long hair you wouldn't even know to look at her that she is blind.
I've considered taking my boy for a walk and leaving her behind but feel bad as I can hear her barking as I leave the house. I do have a very big garden and they both get plenty of exercise chasing after foxes and squirrels. Just don't know what to do for the best really. Oh she's had a full health check too and nothing wrong with her legs - think she has a severe case of lazyitus!