many of the video and audio conversion and almost every video and audio editing program can do that, if there is a mp3 encode codec on the machine like "lame". I would use gold wave http://www.goldwave.com/release.php they let you try it for a while, and it can batch that process along with filter batching. It should be able to handle 2-4hour files.
Super the video converter can also do that, it might not need a MP3 encode codec, just turn off the video, drop one or many files in, and push the button. (super comes with junkware that should be avoided on install) Super can also do video convert.
Many video editing programs can "export" the audio seperate, and do not have to have a video nessisarily, Just like Super any video converter that has some parameters could probably do it, like avisynth, and ffmpeg. Mabey even windows movie maker would allow that?
One for all, on one side having a program that has to many features is not easy to use, but if you already have something on the machine for doing "Video with Audio" it might be able to do audio only, fast and easy. it may just need a MP3 codec.
Searching here on this site for "mp3 convert" comes up with many usefull suggestions , because many of the programs can easily take a PCM uncompressed wave input, all they need to do is the MP3 out.
Program to convert flac to mp3?
How to convert .ogg to .mp3?
Convert mp4 to mp3