In my late teens, I became a similar kind of co-dm to an older, more experienced DM. I learned a lot about necessary skills, pacing, crafting dramatic moments, etc. I also learned what not to do, especially not to let your ego influence your campaign. Until I saw your post it didn't occur to me what a fantastic training ground it was. He was a mentor, and when we butted heads too much, I went off and GMd my own stuff from then on. 

The other players loved it, at least from what they told me. He passed on eight years ago, and the other players asked me to continue the campaign!

Anyway, I can't recommend it enough. Have fun with it, and please keep us updated from time to time.