I'm with you, CC: won't back out of an obligation. Good for you for looking at it as a challenge.
Hopefully they band will be appreciative. Fortunately that's how it turned out after my first sub with a guitarist friend's "casual" band. The setlist doc he provided was hundreds of pages containing over 120 songs with no index. It was all chord/lyric sheets which I despise because my brain works with numbers far better than letters. I said "hey, can you pare the list down to the 60 I'm familiar with?" He said "we'll just call 'em out and you can veto". At the gig I did exercise my veto power, but a few of those times they'd say something like "this one's easy: key of D, mostly 1-5-6-4, just follow us". Turned out to be a fun gig, and I began subbing with them regularly.
On the other hand: one of my favorite drummers asked me to sub with him with a band we'd never played with before. BL was late providing a set list. After some pestering he sent a list of 28 tunes, which didn't seem like enough. At the gig he regularly called out songs that were not on that list, and many of those were not low hanging fruit. It was frustrating: I'm not much of a guitarist and couldn't follow all the chords even when the BL decided to show me his fretboard. Would have been easier if their regular drummer was on board, but of course my drummer friend was pretty much in the same boat as me, so we couldn't often cue each other. Here's the kicker: near the end, they called out "Ventura Highway" by America. This had been brand new to me but was on the list they provided so I had a chance to prep for it. Joe Osborn's bass line is intricate, elegant, unusual... I didn't learn it note-for-note, but emulated his style at the gig. When the song ended I was pleased, but the BL asked if I was playing in the wrong key so I had to explain myself. Turns out they were somewhat of a jam band: the regular bassist insisted on doing his own thing, never emulating the recording, so that led to the BL's confusion. After that gig the BL was clearly less than thrilled, but it was fine with the drummer and me that we wouldn't have to turn down any offers from him in the future.