I felt the same last year when my DD started school, and this year I had DS1 start school too. I could no longer just float about and do what ever. I had to plan around DS2's nap time and actually have a routine, which I hadn't ever really had to do with my kids.
You will find a new normal. You will work out a new routine and figure out how to plan your day around dropping off or picking up kids.

I have two kid free days this year, where they all go to day care/school, so I use those days to go for a walk/swim/gym after I drop them off, then come home and one day I do what ever I want (read all day/watch movies/hobbies/see friends), the other day I spend doing chores I wouldn't normally do with the kids around, like cleaning the oven, or scrubbing the shower recesses. If DH is home, then we go out for lunch or brunch together and have time alone together. The other days DS2 and I go to playgroup one morning, then the library story time another morning, and usually have a play date with friends another morning. One afternoon is going to the park when we pick up the big kids, one arvo is dancing for DD/acrobatics for DS1 and the other arvo is just quiet home time to be together. Weekends are family time and special outings.