I'm an old dinosaur who doesn't "do" the latest computer wigits & flippits, BUT I did discover an "old fashioned" way of doing it on Google Earth.
You just go into Google Earth, then look on the toolbar and it will give you a little icon that says "paths", you then go into this and it gives you the option of "create path". So then you have to fiddle & faff a bit, and work out your hack manually using the little "drawing pin" things, but you CAN work out roughly how far your hack will take you. You can do it in Km's or miles.
Sorry I've not explained it terribly well, BUT its so simple even I can do it!!! No doubt there are websites/programmes that will do it better/more accurately, but this will do for me! And its free.