How long can 0-10V wires for valve control be?
If you use twisted-pair cable, you should have no problems.  I would even go so far as to use shielded twisted-pair cable but I'm an audio guy, so I naturally think that way.

If the person who designed the electronics in the valve was even remotely competent, induced AC mains pickup in the control signal should be either filtered or integrated out and would normally not cause any problems.

To figure out what the voltage at the end of the wire would be, you need to know the DC resistance of the cable as well as the terminating impedance / resistance.  Because you say that the valve controller presents a load of 100k Ohms and you are using copper wire, you can safely assume that the voltage drop in the cable is not measurable and you will see whatever voltage you apply at both ends of the cable.

If you are running the control cable parallel to a high-voltage AC Mains cable, using shielded twisted-pair cable should eliminate any possible induced pickup.