1) Light Source has a "Max distance" parameter, which defaults to 1e6. This needs to be significantly larger than the distance to the light source. For a light at distance 2e7, try a max distance of 1e8. (Ah, now I see you did in your second screenshot, but not the file you uploaded.)
2) Because of the inverse square falloff, light sources at such a large distance need a lot more power. For a light at 2e7 distance, an intensity of about 1e15 or 1e16 gives you a fairly bright landscape. (A useful mental shortcut is that squaring a number in exponential notation involves doubling the exponent, so 1e7 squared is 1e14.)
I've just tested this in 4.5.71 by modifying your file. The behaviour hasn't changed from earlier versions.