Do you need to sing each significant melody in your piece?
I think you have to, if you want to play expressively.
Some pieces don't lend themselves naturally for singing but Widor Andante Sostenuto is a notable exception.
Not only you hear a beautiful melody in the soprano but usually the pedals have this surreal counterpoint in the right foot.
Learning this piece was not so easy. The left hand repetitive double arpeggio accompaniment gave me a lot of trouble.
Sometimes I kept hitting the wrong notes in the extreme right edge of the pedalboard.
I don't know if you can sense it at 2:30, but the registration change to pull this Principal was problematic.
On top of that, the right foot needed a few high f's with the pedal coupler which didn't work for this note.
So in addition to all the thickness of texture, I had to press f' on the third manual with my thumb.
The thing that saved me from panicking and being overwhelmed was of course to remind myself to sing each line.
This singing and breathing at the end of phrases gave me the necessary air supply to finish strong.
I hope this will help you too (and your listeners will marvel at your ability to make the organ sing).