Although I like to have my day organized and planned and hope it runs smoothly, as a mom to 3 young kids it really helps to stay flexible in my schedule. Instead of sulking over the missed opportunities, make new ones.
The day your kid is too sick to head to school, so your planned run/gym time is no more - exercise at home, or clean the house.
You will still have a sick kid, but you will have a clean bathroom, or gotten your exercise in. Not bad for what seemed like a "wasted" day.