I "sing" hymns when I portage.  Always have.  But I sing them silently.  They are the cadence to my steps.  I have several favorites, and have all of the verses memorized, so that it doesn't get too monotonous.  Some examples are "Christ is the King, O Friends Rejoice", "On Eagles Wings", and "The First Song of Isaiah" (which is sometimes called Canticle 9 and is a setting of Isaiah 12: 3-6).  These are slower, for a more "slogging" pace, but the texts are very encouraging.  When I was younger, I used more upbeat marching style hymns like "Lead On, O King Eternal".

So. . .it isn't that I don't sing.  I always sing!  You will just never hear me singing in the wilderness!  I like to think that God hears me, and the cadence encourages me to keep my feet moving.  The outward silence still allows me to hear the rustle of a squirrel (or a moose/bear?) in the woods, or the song of the white-throated sparrow in the trees.  Accompaniment to my inner song!