Lines 6-12 Summary

Get out the microscope, because we’re going through this poem line-by-line.

At this stage of the journey
I would estimate the distance
between myself and my own feelings

At this point in the flight, the distance our speaker feels between himself and his emotions is… (for dramatic conclusion, see the next line).
From the way he says "at this stage," we get the feeling he's been flying for a while.
And there's that word "distance," kind of like a confirmation that that's a big part of what our speaker's trying to talk about.
Only now we know for sure that distance is not just a physical thing for our speaker. He's also talking about distance from his feelings, an emotional distance
It's a small thing, but we notice that our speaker has used the word "journey" instead of, say, the more exact "flight." This word choice could suggest that our speaker wants the word to refer not just to this specific plane ride, but perhaps his entire life. We'll have to keep reading to see if that possibility holds water.

is roughly the same as the mileage
from Seattle to New York,

Our speaker feels very distant from his own emotions—about as distant as Seattle is from New York, apparently. So, like 3,000 miles? That's pretty stinkin' far.
If he's flying from Seattle to New York, and he's above Kansas, then he's right about in the middle of his flight.
That sense of being caught between two places seems fitting for our speaker and for the way he's disconnected from not only the earth, but his own feelings as well.
Plus, you know, he's trapped on an airplane.

so I can lean back into the upholstered interval
between Muzak and lunch,

Our speaker leans back into his seat. He's just listened to some generic music (probably on the airplane radio) and has a little while to wait before lunch is served (this must have been back in the golden days of free food on planes).
The way our speaker blends the image and feel of his chair with the interval of time is pretty awesome ("upholstered interval"). It's kind of like the way we really experience things—we don't see something, then separately feel something, then separately hear something… it all kind of squishes together.
These lines also tap into that same feeling from lines 9-10, of being stuck in the middle between two places. But this time he's stuck between two events, or two moments in time.