Kicker and Daisy......I totally understand and I do the same thing. My boys are grown.....27 and 23.....but they forget that Mom's slow and sometimes I get left behind. Literally!
When they get way ahead of me I just continue walking at my pace knowing they'll eventually look back and realize that I'm not there!

That's what it takes for them to understand. Not me pushing myself to keep up....but me going at my own comfortable pace and them slowing down to stay in step with me. I tell them if they want to go faster I'll catch up but I can't do the marathon walking through the mall anymore.
In a way I'm glad they don't understand.....I just want them to be able to do the things I can't. If someone has to have this crappy disease I'd rather it be me than them.