If it doesn't have a remote ground connection, and its insulation hasn't been compromised, then there's only the equivalent capacitance of cable to ground, loading it in the common mode. That's what, 30nF total? (~30pF/m being ballpark typical.) And it's a transmission line so for brief impulses (if we take a typical induced lightning waveform of the 8/20 or 1.5/50us type, compared to the >3.3us electrical length of the line, we should expect some TL behavior), the source impedance corresponds to the transmission line characteristic, so, some hundred ohms give or take. Which, if this is buried, then the impedance may be lower actually, but also very lossy because earth forms the opposite electrode (so we expect lots of losses at high frequency, further rounding-off the impulse); or if suspended in air on poles or whatever, then a higher impedance (~200 ohms?) as wire over ground plane (ground again being earth, but with some air inbetween, the frequency response may be fair). And for intermediate cases (wire laid on ground? where rats/chipmunks can gnaw it??), something inbetween. Also, an elevated line may be subject to additional electric fields, because, line of sight and all that.
So, I guess we could potentially expect quite a high voltage at the source (multiple kV?), but the result should be that it has quite a high impedance (low 100s ohms) at much of a distance, so the peak current isn't devastating, pretty ordinary fare for TVSs and such. Having a grounded receiver is no problem, and all the transients can be filtered out. Presumably the sensor's bandwidth only needs to be fractional Hz, right? And it's not sending digital code or anything?
If that assumption (no grounding) is violated, then the signal will both become imbalanced (also violating the current loop assumption), and the source impedance may drop (having a direct connection, maybe induced lightning is able to spark into it instead -- speaking of, sparks act to sharpen a waveform so there may be more high frequency content in this case). Isolation may be required to deal with this.
Also, if the cable is 24 AWG, it'll have a good 160 ohms total loop resistance. Pretty big, but well within the compliance range of typical 4-20 receivers. If we expect the average lightning strike near the middle of the run, that's half or 40 ohms per line as the source resistance, plus whatever impedance it has due to environment. This sets the minimum, and again isn't much of a problem -- most protective devices are suitable for say 20 ohm surges, a typical test level for industrial and ITE circuits. (Or, if we take the equivalent of both lines in parallel, that is indeed 20 ohms common mode.)
Tim