From Halopedia, the Halo wiki
Walking Away is the eighteenth track (Disc 2 Track 11) included in Halo: Reach Original Soundtrack.
The track opens with snare drums, followed by brass instruments playing the melody of A Walk in the Woods (from Halo: Original Soundtrack) at a slower tempo. After the melody’s first phrase, the brass instruments stop briefly, allowing a drumroll by the snare drum. The melody repeats, this time joined by vocals, bass guitar, and a drum kit. The melody repeats for a final time, albeit without brass instruments. The piece ends with the bass and vocals.
Walking Away is a bonus track found exclusively in the soundtrack, and thus cannot be heard in any part of Halo: Reach. However, it can be heard in Bungie's ViDoc O Brave New World, during the credits.
The song's name is most likely named after A Walk in the Woods and a reference to Bungie finishing their work on the Halo franchise.