Pete,
First, I just measured your trail from the inline with Meadow Ln. on the trail to the NE end of the Kokosing Gap Trail, using ArcExplorer. ArcExplorer is a free viewer from ESRI, makers of high-end mapping software. You may download ArcExplorer free from their Website. HOWEVER, I have not gotten it to work on my personal computer or laptop, due to video driver issues. Have tried to resolve them, to no avail. No guarantees on your computers.
That said, the distance from Meadow Ln. to the NE end of the trail is 8.842 miles, give or take a couple hundred feet.
Your sketch shows runners doing almost a complete lap on the track. The track is .25 mile, if it is a regulation track. Sketch also shows Mile 26 out on the trail. Now, in my book, the Marathon is 26.21876 miles. So, the sketch has extra length in it.
BUT, given that I ASSUME that it is 8.3 miles from the Start to Meadow Ln & Trail, AND, if Mile 26 is actually closer to the track than the sketch shows, you SHOULD have ample room on the trail for your spur.
So, I must disagree that you don't have use for a co-rider. My method:
Take co-rider (and two cars) to Cal Course and lay it out. Calibrate.
Drive one car (with both riders) to the Start. Lay out your mile markers as you ride to Mile 8.5 or Mile 9 (point is to get beyond the trail junction). Should be good as gold.
Both riders ride to Finish. From Finish, ride backwards (mark Mile 26 when you get there) to your stopping point (Mile 8.5 or Mile 9, whichever you chose). You now know (after doing some math) exactly how long your spur to the northeast needs to be. Ride to the turnaround and back, marking your miles from Mile 9 thru Mile 25. Finished! (As long as your two rides are within tolerance, and you have been marking the "longer" measurements.
If I did it myself, with no co-rider, I would calibrate, drive to Start, ride Start to Mile 9, then find distance from Mile 9 location to the Finish, so I could calculate how long my spur needs to be. I would mark Mile 26 while there. Now, I'd ride back to Start to finish the measurement of the first 9 miles. I would adjust the Mile 9 point if an adjustment is necessary.
On my second day, I would ride and mark the spur, as I do believe there is plenty of room, especially if runners make an entire lap on the track.
I have started doing my Marathon courses with a co-rider, and that is much better than alone. Although, with the trail, you don't have to worry much about traffic, except through that short stretch in town.
Good luck. Wish my Marathon courses were that flat!