The speaker admires the speed and stamina of – what exactly? She only says that she likes to see "it" as it travels.
The double-edged verb used here, "lap," immediately brings to mind two different actions. The speaker could either be describing a racehorse, flying through the laps of a racetrack for mile, or a cat, languidly lapping up miles like milk.
Either way, something is moving across a landscape – fast.
If we remember the alternate title of the poem, "The Railway Train," we can already guess that she's probably watching a train moving along the tracks.
And lick the Valleys up —
Going off of the second meaning of "lap" in line 1, the image of the mystery animal "lick[ing] the valleys up" follows on this theme of eating and consumption. "It," the train, is eager to eat up (metaphorically speaking) the distance it covers.
This use of the word "lick" is an example of personification. The train is described in human terms, because it "licks" the valleys: it's as if the train has a tongue.
Trains can't actually lick, of course, but it's a strong image that gives us an idea of how the train moves through the valleys.
And stop to feed itself at Tanks —
Another eating-related metaphor appears here, as the mystery creature "stop[s] to feed itself at tanks" (3).
Trains, back when Dickinson was writing, were all powered by steam, which was generated by burning wood, coal, or oil.
So, this must be a reference to the train being loaded with fuel so that it can keep moving.
Let's keep chugging along and see where else this train goes.