How to know what this impedance is?
Your hypothetical model is more complex than the data calls for. It sounds kind of like an RIAA equalization filter, which is just two resistors in series, with a capacitor in parallel with one of them. At very low frequencies, you see just the two resistors in series. At very high frequencies, you see only the resistor that isn't shunted by the capacitor. In between, you see an "S" curve whose exact shape depends on the relative values of the resistors and the capacitor.

Since your circuit has an impedance that rises with frequency, change the capacitor to an inductor.