Hi,
What software are you currently using to convert that WMA file to MP3? I ask because, though the file size might change depending on your choice of bitrate and suchlike, the lenth shouldn't be cut down at all.
You're definitely heading along the right lines by trying to convert the file first, as there are so many more tools that can work directly with mp3 format.
With a file this big, I'd start by copying it off of your Olympus and working with it on disk first of all, access times will be faster this way and you're less likely to run out of space half way through the job.
Software-wise, one free option that did a good job of transcoding a ton of formats last I looked is Foobar2000. You can get it from
www.foobar2000.org
Once you've got it into mp3 format, if you know the rough time range of the part of the file you want to keep, you might be able to save some time by using a file splitter to chop quickly down to that part and work on tidying up that small chunk instead of importing the whole 13 hours. There are a ton of free mp3 splitters that will come from a Google search, I use a simple one called MP3Clip for quick and dirty splits like this.
I haven't seen your particular model of Olympus, but for future reference, if it is possible to record directly to mp3 it'd be worth doing. WMA has absolutely no advantage in terms of sound quality other than at seriously low bitrates, and as you've discovered, it's a pain in the ass to tinker with afterward!
Hope that helps a bit. Feel free to ask anything you get stuck with, no question is too big or small here man
Sc0tt-uk
PS. Out of curiosity, what's the action you're trying to get at here? I'm wondering what we've got to look forward too...