First of all, you decide you'll do your morning exercises later because you woke up with an inspiration about a scene/character/chapter/event in your story that you simply HAVE TO WRITE before you forget. So you start to write and then you forget to stop.
Your tummy rumbles and you remember that you forgot breakfast, so you go into the kitchen and think about all the nutritious breakfast food you have that will help your brain function and therefore make you a better writer. But you settle for a diet shake because it's instant, you can down it in three seconds and get back to your writing. Then you think about a change in another part of your story that needs to be made to fit in with that inspiring bit you just added, so you start to work on that.
Your husband comes in to the room to tell you something and you scream because you were somewhere else, not here in the real world. He goes away. Back to that other world you love so much.
Your feet start to feel cold because you haven't done your morning exercises and therefore haven't moved for three hours or four hours, so you haul out the heated mat that goes under the desk for the duration of the winter months (even though it's only early autumn right now) and turn it up to high heat. Lovely.
Writing is progressing at a rate of knots with no effort at all. In fact, I can hardly keep up with my own brilliant thoughts. Then the tummy rumbles again. Damn. Gotta eat. OK, back into the kitchen, husband is watching the golf with a delicious salad sandwich in his hands. I think that's what he came into the room to ask me earlier - would I like one too? I should have said yes instead of screaming. I should make one now. Nah! Too complicated. Slice of cheese between two slices of bread and back to the keyboard.
Need to develop a character detail to tie in with the earlier inspiration. Shoulders begin to ache. Ignore that. Slight headache from staring at the screen for too long. Fingers getting stiff for so much typing. Ignore all of that, too.
OMG! It's getting dark outside! How did that happen? And there's a funny smell. Oh, I forgot to shower. OK, better get up and do that or my husband won't want to come to bed tonight.
And here's how you REALLY know when you've been having a good day of writing. When you get up, you have CORNERS ON YOUR BUM! Anyone know where I can buy a pair of square-bummed trousers?