1) Make sure the correct pairs are twisted.
2) Make your oou use own cable. Will you use eth and serial at the same time? Then you could just make a y-cable making sure yourself that power doesn't enter the NIC. If you use only Eth, make a cable where the serial lines are broken.

Interference won't be a problem at these lengths of cable.
For instance BreezeAccess (www.breezecom.com) outdoor-solution uses these signals in the same cable.

MArius (Escort cab + MarkII)