Just make mp3 files bookmarkable with a tag. No AAC conversion necessary.
MarkAble places the files into iTunes Library and directs iTunes to convert the files to AAC (this is what takes so long), and after doing so, renames and performs the file management necessary to place the converted files properly in the iTunes Library.
Since you have mp3 files and if you have a clickwheel ipod, use the markAble make mp3 bookmarkable process for click wheels only. It is quick and easy, giving you the choice of a full AAC conversion or mp3 bookmarking tag at the "convert all" step. This will avoid the lengthy AAC conversion, but the resulting files will not list in audiobooks main menu, will not play faster or slower, and will not be skipped in main menu shuffle. -- problems acceptable to most and some of which can be over come with smart playlists.
Jonboy suggests doing a right click on the mp3 files and changing ID3 to none to avoid problems with mono and long files and reports no problems with mono files and up to 26 hour files. Moriond thinks that selecting an older version of ID3 may do the trick. You can do this on an mp3 file before or after conversion to bookmarking if you do not choose the full AAC conversion. In iTunes Library, Right click the mp3 file (or a highlighted group of mp3 files) and select convert ID3, and choose none or possibly a lower numbered v if you wish to retain the tag.
Please report back if this works,and which worked (none or lower version), to eliminate freezes (lockups) and bookmarking problems on mono and long (over 5 hour) files.