Combining mp3's results in a file twice as long

The first thing here is that Audition doesn't combine MP3 files. It doesn't do anything with them except decode them to wav files which it can open - this is Audition's internal working format, and it's what happens with any compressed file. But even allowing for that, 44 minutes seems to be rather a lot of extra time - how long were these files in the first place? And when you play them back, has the pitch altered? Most importantly though, are you saving the resultant file as a VBR or CBR? Saving as a VBR means that any indication of the actual running time is a guess, as the re-encoding with its resultant coding gain hasn't happened yet, and that's a variable feast. Yes, if you do anything to MP3 files in Audition they get decoded and re-encoded, and there's inevitably a quality loss.

But basically, if the pitch hasn't altered, then the real run time (as you'd measure it with a stopwatch) hasn't altered either. What we need here is to know a bit more about your files, and exactly how you are joining them. Are they all the same bit rate/depth?