As discussed by Tim, you have to model your coaxial cable as a 50 Ohm transmission line (at least I hope you are using 50 Ohm coax). There's a bit more to the story however.
The question is what the input to the scope looks like. If you are using a scope that has no internal 50 Ohm termination and you add your own terminator externally, then what you have is more or less correct (we are ignoring the extra transmission line parasitics from having the terminator external to the scope). Also, some lower end scopes with internal 50 Ohm terminators also simply put 50 Ohms in shunt with the original 1Meg/16pF input.
But this arrangement causes an inherent bandwidth limitation. Suppose you considered the ideal scenario in which you drive the scope input with a 50 Ohm source impedance but without any cabling between the source and the scope. You would essentially form an RC network with an RC time constant at RC/2, where R is 50 Ohms and C is 16 pF. The pole sets an inherent bandwidth limitation at around 400 MHz for these numbers.
For a well designed high frequency scope, the 50 Ohm internal termination will not have 16 pF in parallel with it. It is perfectly reasonable to expect well less than a few pF of capacitance with modern input stage amplifiers, but mileage will vary depending on the scope you are using.
Taking all this into account, you have to revise your circuit model. The 1 Meg resistor is inconsequential, the 16 pF capacitor should be an order of magnitude smaller, and, as per Tim's answer, the coax should be modeled as a 50 Ohm transmission line (NOT a lumped capacitance) with length corresponding to your situation.
Regarding your question about transmission lines, an ideal lossless 50 Ohm transmission line looks like 50 Ohm all the way to DC. In practice, the finite loss of 50 Ohm line results in a slightly higher impedance at low frequencies (and it actually becomes dispersive), but those effects are fairly negligible in your situation.
50 Ohm line can look capacitive or inductive if the electrical length of the line is shorter than roughly a quarter wavelength, as mentioned by Tim. This property is a manifestation of multiple reflections and used to construct stepped impedance filters. If you properly terminate a 50 Ohm line in 50 Ohms and you drive the coax with a step, you will not see an RC exponential settling at the load -- you will see a step.