If your source recording is already split into tracks, you just have to transfer them to a computer, and use Sonic Stage to transfer to MD (provided your player is a NetMD or HiMD).
Otherwise you're left with analog transfers, which takes longer and degrades quality.
If your source recording is not split, I'd split it on a computer first anyway.
If you really want to transfer directly from M10 to MD, not going via computer, you'd have to split the file on either device and go analog in realtime.
In that case, I would personally just drop $25 or so on a used Sandisk Sansa instead. They support Rockbox, and you could simply pop the microSD card in and listen to your recording:
http://www.rockbox.org/