Lumped parameter vs transmission line model for a piece of cable
The speed of propogation in a 'normal' coaxial cable, that is straight cylindrical conductors with an annular dielectric spacer, is entirely controlled by the dielectric constant of the spacer. For most solid plastics, this a bit more than 2, so the speed in typical RF cables is 0.67c, regardless of the other dimensions. Foamed or other air-containing spacer structures will increase the speed closer to c.

5m of cable has an electrical length of about 8m of air, about 25nS, which at 1MHz is 8/300 = 0.027 wavelengths. 

This is less than the \$\lambda/20\$ or so that we normally regard as 'short' for a transmission line, so it would be quite reasonable to represent an open line as a lumped capacitance, or a shorted line as a lumped inductance, or the line itself as a ladder of Cs and Ls. 

You could design the LC ladder by simply using the rated impedance per length of the line. The smaller the individual Cs and Ls, the better will be the approximation. An alternative is to design a low order lowpass filter, either CLC or LCL, choosing the bandwidth such that it gives you the right phase shift at 1MHz.