Hi Ann,
I've been thinking for a while about how to reply to this. My Mum and I both have underactive thyroids, so I do know a bit about it. Problem is that it can become very easy to blame everything on not being balanced. It is probably worth this lady getting a thyroid function test at the doctors, but not to be surprised if it comes out normal. Once you are balanced your metabolism should go back to normal, and your thyroid shouldn't affect you. I guess it is a bit like being short sighted, once you have the correct glasses you should be totally 'normal' again.
There have been a few threads where people have become stuck at some distance or another. Maybe try to search for them to get some other ideas of how to help. I find if I try to trick myself it can help. The way I do it is to go on a different route and then I don't notice how far I'm running. Or if you are running in a big group, get a really interesting conversation going at around 6 miles, that might distract her from the length of the run! :-)