That's "Childe Roland to the Dark Tower Came" by Robert Browning. The lines you're thinking of are these:
For mark! no sooner was I fairly found Pledged to the plain, after a pace or two, Than, pausing to throw backward a last view O'er the safe road, 'twas gone; grey plain all round: Nothing but plain to the horizon's bound. I might go on; nought else remained to do.
Re: The path poem
For mark! no sooner was I fairly found
Pledged to the plain, after a pace or two,
Than, pausing to throw backward a last view
O'er the safe road, 'twas gone; grey plain all round:
Nothing but plain to the horizon's bound.
I might go on; nought else remained to do.
The rest of the poem:
https://allpoetry.com/Childe-Roland-To-The-Dark-Tower-Came