You have two options: Just retrieve the current Kepler elements by using "GetElements()". And work with the values of these. Or calculate these elements yourself by using the state vector (velocity and position relative to Earth).
When you have the elements, you can calculate apoapsis and periapsis distance. The next step for apsis distance is then subtracting the planet radius below each apsis. Thats simple for a spherical abstraction. And a tiny challenge when you use a WGS-84 ellipsoid.