I've ended up rolling a much nicer, more fun world in the new game. Starting out spawning on top of a giant tree - next to an ocean, so I could jump offIf you want to recover your old world, you can use an external NBT editor to change your position back to the overworld.
Also check to see if the pack comes with Not Enough Items, you would be able to switch to creative mode temporarily and get some obsidian.
When I was playing with Mystcraft, the instructions did not say that books were instantly destroyed by rain. So I learned a few things.
I have NEI installed, BTW, but I couldn't get it to give me anything creative-mode-like. I could switch it to "creative" and select something, get the stack of 64 of that thing, but when I tried to put that in my inventory it just disappeared
Mystcraft instructions suck ass. It took a wiki page and then a youtube video before I could figure out how to use the stupid writing desk. Personally, I think it's unnecessarily unforgiving, but it does offer some great exploration.