So that would mean starting at the TTC, running to MK and back. All well and good but they would have to drive to the TTC to start their run. I agree it would be good to know the distance.
However someone staying at the Contemporary or Bay Lake could do the run to TTC and back, which would be more convenient. But it entails going through security near the start. It isn’t clear whether going early enough in the morning, say 6am, allows you to get through or if you get turned back (which some people have reported in the last or so).
So I just mapped it. If you ran it the way I would (which means taking the paths closest to the water wherever possible and prudent) and also adhering to sticking to sidewalks instead of going on roads (which Disney has issues with) you get a 2.5 mile one direction. With the run starting from the very end of the sidewalk south of the water bridge and going all the way around through MK security and down the (once completed) path to TTC security. Turn around and the out and back is a 5 mile run or 8k.
If you wanted to continue from TTC onto Seven Seas Drive and then World Drive and make it a loop, its 3.4 miles, but there aren't shoulders on either road and Disney doesn't like runners on the roads unless you're paying them.
And no, you wouldn't have to drive anywhere. If you're saying at say GF, you would run either way to the turn, back past GF, to the other turn, and finish at GF. Just because you set an 'end' doesn't mean it has to actually be where you start/finish.