The Aging Agency is there to help you, your family, and your MIL, not just financially. So even if she's not eligible for certain subsidized programs, she still can participate in activities and you can still get caretaker support (even if it's just talking to others in your position and learning about your options!). The Agency is not just for people with low incomes. Ask them what options are available for you and for her. They are the professionals in this field and that's what they are there for!
Don't get me wrong--there's nothing wrong with being a SAHM. If you can't work in exercise now, then it would be harder if you are working, unless you had other supports discussed above, but still it's adding another layer. I just meant, you know, down the road, if you wanted to work in something that interests you, that you should look into it and not let things keep you from pursuing your dreams. And I meant working part time while the DS is in school, that way you would be home when he's home. I agree on that wholeheartedly.
You are to be commended for all your hard work and patience. I hope you can work in time for yourself soon! I couldn't stand to be with my (now ex) MIL, much less have to take care of her!
Have a good bike ride!