I have been unable to discover anything about isolated conductor capacitance. One approach would surely be to find the formula for an air dialectic coaxial capacitor. The denominator of this formula is the natural logarithm of the ratio of the coax external and internal dimensions.
If we now set the dimension ratio to some large number, say 10000 then we arrive at an approximation of a few picofarads per metre.
Is this a realistic way of estimating a result?