If 20mA = 5000mm, the Arduino 10 bit ADC (0 to 1023 counts) will only get a resolution of 4.89mm per count. You need an external, 16 bit ADC module like ADS1115 (65535 counts), and choose a load resistor that would give an ADC count of 62500 at 20mA. Update time of 100ms should be doable.
https://www.adafruit.com/product/1085