The title borrows its opening words from the famous line about being midway through life's journey, and the poem seems to start from that same moment of looking at how much time has passed. Instead of Dante's dark wood, though, the speaker sees time as a "thin woods": something sparse, easy to see through, not much cover and not much left.
The odd part is the speaker's promise. "I am sure I shall save / for one night" is deliberately unfinished. We never learn what will be saved, and the missing object makes the line feel wistful. Judging by the images, the speaker hopes to hold back a small piece of time, maybe to enjoy it fully, but the confidence sounds a little shaky. One night is a tiny thing to set aside from a whole life.
The mood is quiet, reflective, and slightly sad, with a thread of stubborn hope running through it.