PCB Trace length vs Impedance
Trace impedance and trace resistance are different things, important in different situations.

For RF work, and for high speed digital, the characteristic impedance of the trace is important, as it needs to be driven and terminated in a way that minimises reflections. This characterstic impedance is independent of length and trace material, depends on substrate thickness and trace width, and is usually in the 50 to 100 ohm range. This is the ratio of voltage to current as a wave propagates down the line.

When you are distributing power, DC and low frequency, the trace resistance becomes important. This is proportional to length, trace material, thickness and width, but not to substrate thickness. It is usually in the milli-ohms range. This is the ratio of the voltage dropped over the length of the line to the current flowing through it.