Thinking outside the box or going out on a limb... a different idea...
First let me tell you about Micah True, ultra-marathon runner who for years ran long distance races of 100 miles. He was out on a simple 12 mile run (alone) where it appeared he simply sat down beside a creek. He died of an undiagnosed heart problem, an irregular heartbeat during exertion.
Might this have happened to M? Being a runner and appearing physically fit does not show what's happening inside. Did she sense a problem, stop somewhere off a road, and expire? One would think buzzard would have located a body or cadaver dogs, but maybe not.
Just another idea because the others aren't working...