To manage my time as I see fit and not report to MIL every five minutes??

[DifferentDay]

Try to keep this brief. I am a researcher/ analysis type professional. I work from home three days a week and that time is spent analysing data and writing up research - very sedentary, but very intense. Mil has dcs either side of school these days, picks them, takes them to school and then picks them up and brings them home again. I don't work holidays and DH and I juggle the remaining school days.
Doing my job, plus having two kids and not making the wisest of choices, I have ballooned in weight. I therefore decided to do something about it and took up running at lunchtime the days I work from home, so three days a week I pop out at lunch (I've always taken a lunch break btw!) and go running (all in all with changing it probably takes 45min - 1 hour. Mil thinks this is out of order and moans that she doesn't look after my kids so I can 'swan about'. I've lost over two stone and she also seems to believe this is somehow because I am unhappy with DH (this is a ridiculous argument and DH and I very happy, regardless of my weight!)
I kind of get the argument if I took that time at either end of the day I could pick up or drop kids off but I don't believe working straight through with what I do is possible either, I just can't do it, my head would implode - and I've never done that! Regardless what I do with it, I need a break in the day. My other sore point in this is that I've argued with mil over kids, I wanted a childminder friend to do drop off and pick up for us, and us pay her, to keep it professional - to avoid exactly these types of problems. Mil was utterly against this and wanted to do it. To be fair to her she often takes kids to park after school etc and they love this time with her, and she is brilliant with them, but every day now she questions me on what I've done with my day, have I been running and I'm finding it a bit annoying. I do see her point but think my time is to manage as I see fit... AIBU

[PumpkinPies38]

Tell her if she isn't happy looking after her grandchildren with the current arrangement you can find a child minder. She sounds an utter bore.