I have tested the Anatek "MIDI to XLR Line Driver" system to ~1000 ft.
using twisted door bell wire.  It worked just fine.  The specifications
suggest that 4000' of shielded XLR microphone cable will work.  The
system consists of a black box at each end, with an associated wall
wart (power supply).  Each box at each end has two MIDI connectors (in
and out) two femal XLR (microphone) connectors, and a couple of LEDs.
It is not necessary to run two pairs of iwre between the two boxes if
transmission in only one direction is required.  I have some of these
in stock for $115 per pair plus shipping.

Mike Ames  619-481-1663