How to get less tired from belting

[Wherethehecisbrandon]

Hi,

My name is Brandon, and I'm currently singing lead tenor in a Barbershop quartet.  The song we've been learning is "Notre Dame medley" by Ringmasters, and while I can sing the whole song in pieces, the part is so high that by the end of the song I've been belting in the D4 to A4 range for a 6 and a half minute song, and my voice is really taking a beating.  This is made worse by the fact that at the very end of the song (the "tag") my part has the post, which means I have to hold out an A4 for 20 seconds straight while the other three singers do other things.  I can get the post fine normally, but after singing the whole song, my voice is straining a bit to stay on the note, and sometimes it goes flat or accidentally flips into my mixed voice (which I haven't worked on too much so it sounds really bad).  I can't do it in falsetto because having it belted is a big part of the ending, and overall it would feel weak(not to mention I don't exactly have the best falsetto).  What exercises can I do to increase vocal stamina and what can I do while singing to make myself less tired at the end?  The video of the original group doing the song is linked below, and the one singing my part is the one 2nd from the right.

https://youtu.be/Qo_N9_ZFBhs

[kickingtone]

I don't think it is a belt, actually.

But dyaaaam! I've never really thought about trying to hold a single note like A4 for that long.

Thanks for drawing my attention to a new exercise with this thread. I've just given it a try!

It's gonna need work, lol! Dreadful! I won't even post my first two attempts.

I think the main key is going to be keeping the diaphragm really steady. Maybe I should actually stand up, too!