I listen to a few podcasts and as long as they have a consistent format, the sky is the limit in my opinion. I listen to one that runs roughly 1.5 hours every day (Adam Carolla) and a couple others that only run 30 minutes (a survival podcast). I would also be consistent with the release dates of each podcast so your listeners know when to expect it. Every monday and friday I know I can get Jay Mohr's podcast. Carolla's (the #1 most downloaded podcast) is a Mon-Fri format, which is likely mostly due to demand. So pick a day of the week to release it, or every other week, etc.
I'd love to hear a podcast geared more toward outdoor stuff I like. It seems most "outdoor" podcasts out there are for the "hunter" audience. It would be cool to have topics on specific hikes and as others said, special guests, maybe outdoor/environmental issues that are hot topics, etc.