Pete,
Assuming the TA is the only marker where the course can be adjusted, then the measurement could be completed in two days. On day one, lay out your calibration course as you had specified, and calibrate. Measure from Mile 8.5/26 marker to the fixed start. Then, ride from the start back to Mile 8.5/26 marker and lay the splits along the way back.
If there's time on the first day, measure the distance between the Mile 8.5/26 marker to the finish on the track. Then, ride from the finish back to Mile 8.5/26 marker and lay the splits along the way back.
At the end of the first day, you'll know the distance from the start to the Mile 8.5/26 marker as well as the distance from the finish to the Mile 8.5/26 marker. Now, you can calculate the remaining distance to make up the marathon as well as the split locations. The turn-around will be located at one-half of that remaining distance.
On day two, calibrate, and measure from the Mile 8.5/26 marker toward the TA and locate the new TA. Then, ride from the TA back toward the Mile 8.5/26 marker and lay out the splits along the way back.
Let me know what you think. -- Justin