I've been using various models of the SanDisk "Sansa" MP3 player for years for similar reasons. Lets me listen to audiobooks/podcasts all day without wasting phone battery, and the controls I use most often (pause/play, forward, back, volume) are physical and can be located by touch alone. Also, durable enough for use during damp/dirty outdoor activities, and cheap enough not to be a big deal on the rare occasions that I do ruin one. Unfortunately they seem to be on the way out (Audible support for my newest model seems buggy and undocumented, and the SanDisk web site doesn't seem to push/advertise them much any more).