When did I lose my ability to say "No"?

“Many fine things can be done in a day if you don't always make that day tomorrow”

By saying "No" I don't necessarily mean to food. My problem of late is saying no to my family. It seems no matter how much I plan or explain to my children they find a way to sabotage my efforts to do the things I need to for myself. So again and again I put it off till tomorrow. I get ready for a class, for a walk, for time for me and at the last second someone needs something immediately and I jump. Now understand my kids are not young. They are 18 and 13, perfectly capable of doing things themselves. Yet of late my afternoon after work (which my schedule was taken so i would have extra time for me to focus on exercise and healing) is spent running errands to 3-5 places before I ever get home. Then on to laundry, dishes, dinner, cleaning, etc.

My husband tell me I need to stand my ground except he is one of the people who hands off errands to me. So how do I say no? How do I make them understand I need time for me and not at 4 am or 11 pm? I am open to suggestions.