«فَعَنَّ لَنَا إِعْمَالُ الرِّكَابِ»
Then the idea occurred to us to press our mounts onward.
ʿAnna lanā means that an idea occurred to the party; iʿmāl denotes setting something to work, and rikāb refers to riding animals or mounts.
This is a joint decision in the journey. The important change is that they will travel at night, opening a new sequence of imagery about darkness, fatigue, dawn and rest.