Char impedance has nothing to do with the lumped impedance of the cable. If a cable (two wires) is treated like a transmission line, it means that the cable is caring a signal (sine wave for example) whos frequency is high enough so the signal's wavelength is significantly smaller that the cable's length (at least x10 times smaller). This makes the cable a distributed system, in which Kirchhoff laws no longer holds (they need to be replaced with a more generalized set of equations, called the Maxwell equations, which are not only function of time only like KVL/KCL. but functions of both distance and time.
The char imp, Zo is a ratio between the magnitudes of the traveling voltage and current waves in the same direction (voltage or electric field. current or magnetic field). To calculate the cable's lumped impedance (not distributed), we assume at the sending end we have V1, I1, and V2 and I2 at the receiving end. The we can take the voltage drop of the cable V2-V1 and divide it by the average current (I1+I2)/2.
Note that the cable has two types of impedance elements. Series elements like copper resistance R and wire inductance L, and parallel elements, like capacitance C (wire to shield), and leakage conduction G (between lead wire and shield due to dielectric contamination)
So, the longer the cable's length, the greater the series impedances R and wL, and the lower the parallel ones, 1/wC and 1/G. So, the total impedance tend to remain stable along the length, which in a way, explains the need for the char impedance - a quantity that is not affected by length of the cable.
One more thing, if the cable length is infinite, then Zo will be the same as the actual impedance along the cable...