Gosh, this has been so enlightening and has probably changed my perspective on what I consider a hobby. I had bracketed it in my mind in relation to the amount of time spent, regularity (groups, classes), whether it's an organised activity, requires some special gear/equipment, a true passion that absorbs people outside of their other commitments and obligations. Thanks for the wisdom from those who have been in my shoes or are currently in the same time-poor scenario.
To answer some of the questions:
I don't just stare at the walls or doomscroll. Often it's 7.30/8.00 by the time I get home after children's activities, so the remainder of the day is taken by talking to the children about their days, helping with homework, putting the younger one to bed, making lunches/packing bags for following day, dinner, shower, bit of life admin. Then it's time for bed.
My house is not spotless. I gave up on that years ago. I do the essentials during the week, and little more than that at the weekend.
I don't think I'm uninteresting or unfulfilled. I have a very varied working life, listen to radio/podcasts during the commute, love to travel and try new experiences, can hold conversations with a wide range of people in different settings.
Mumsnet as a hobby is an interesting one - I would say it's not, as it is usually just killing time in an awkward gap between meetings or waiting to pick up a child. Probably adds up to 20 mins a week, maximum.
Things I do regularly, including gym and walking, I had not considered as hobbies but maybe perform the same functions as others have listed - time to myself, decompression, distraction.
As a child I was given lots of opportunities to try activities (piano, tennis, dancing, local theatre group...I'm sure there were more). I didn't hate any of them, but never discovered a passion. I don't think I'm good at anything - I'm not musical, artistic, sporty...
Reflecting on everything that has been shared, maybe a way to frame things for myself is: If I had a week off work, unexpectedly, no obligations and with no judgement from others, what would I do?
I would probably do some travelling, exercise, reading, cooking/baking, maybe a spa day, coffee with friends. So perhaps these are my hobbies, or would become my hobbies in the future as and when time becomes available...