So, if I understand correctly:
1) both rheostats are at the console,
2) each rheostat has only a 2-wire connection back to the sealed unit (ground being one wire?),
3) the 10k rheostat controls current-limiting,
4) the 20k rheostat connected to pin 5 controls speed in some way, but is not the main speed control? I'm puzzled as to why there are two speed controls.
Is the 10k a factory-preset or a user-operated control?
Is the 20k a factory-preset or a user-operated control?
Can you post a link to the treadmill manual or spec?