I admit, the first writing session on a new story can be a rush that no later session can match, because for the first you don't have to pick up the threads where you dropped them. I try to make sure I start it as early as possible so I don't have to cut off the rush of words for something dreary, like, say, sleeping.
But even there, and much more often in later sessions, it's not a matter of sitting down when inspired but sitting down, writing, and then being inspired. I set my first quota, of a page, because I found that after a page of writing, I was warm to my work and often would sail for hours and pages more. Of course, I had to sit down to write first. Which can be interesting. No matter how much you know that you are merely vacuuming the cat, it can be interesting.
Then there's running out of steam when you have poked at the story and tried to work on it. When it's at the end of a session, I figure that the well had been drained dry and leave it to refill for the next day. But when it's at the beginning. . . I usually go off and work on something else. Which is very bad advice, because it is ever so fearfully easy to go on and on and on to new pieces and never circle around to see if the well filled up again. It often does, if I circle back to it, but it's a hard habit to pick up.