I agree with the couple of replies you've already had --- audio books on the trail can absolutely make sense. I listen to book when I'm doing relatively long hikes by myself.

What works best for me if I plan to listen a lot is a small standalone MP3 player, one that takes a single AAA battery. The great thing about a AAA battery as power source is that you can estimate your amount of listening ahead of time and, at about 18 hours of listening per battery for me, plan out ahead of time how many very light batteries to carry. All without stealing power from a smartphone that in my case at least is used for more important things (photos, blogging, occasional GPS, emergency phone use).
Brian Lewis