It won't GET HOT, but once you have it hot, it won't be subjected to the room air while it's in that tube.
It'll hold its heat for a while.
With a 12 inch forge, and proper fuel/air supply, like Lin and I have, where you run high pressure from the tank, and then tweak it down with a needle valve at the forge, and PLENTY of air, you'll heat up more steel that you can move.
And it you're making Damascus, you want to forge it HOT, and if you heat up 10 inches of steel, it'll cool off before you can work it anyway.
No reason to heat up more steel than you can move while hot.
Work the end first, then run that into the extension tube and work the next section.