Most RD here want a common start and finish point to save the extra cost of the timer setting up two different points or moving mats, so that would seldom be an option here. As Bob stated, there are other reasons a loop plus might not work. Mile points would also never be correct, if they found them. I measured a 5k at O'Hare on a new runway that was not active, but I had airport security with me all the way. I also measured one at Chicago Executive Airport at 3:00 a.m. and had airport maintenance with me who were broadcasting on the ground frequency at every turn. I did not have any problems making smaller paint marks for the start, finish and mile points for both courses. Look or ask for a "Movement Diagram" of the airport. All runways, taxiways and pads are identified and marked and you should have no problem finding landmarks. I am a pilot but I used the airport maintenance supervisor to help develop the description and reference to the landmark, so the lingo was correct. If it works, the attached URL will take you to the sample diagrams for Chicago Executive Airport. The course I measured there is IL18011WR.
Chi Ex Airport Diagram