Rojo, if you're going to put up money for a challenge, I suggest that the distance be 5k and his time to meet would be his first 5k split time at the 2019 LA Marathon... 20:02. In your challenge, you should stress that 5k is less than one-eighth the distance of a marathon and that any legitimate elite marathoner could do a 5k at their current marathon pace as an easy training run. Certainly, any legitimate runner would be willing to run their marathon pace for 20 minutes on a track for a $10,000 donation to their favorite charity.
Require that it be on a track with 3rd party observers and a representative from LetsRun.
The reason I suggest a 5k is that this is now about optics and public opinion. Frank can delay and then claim an injury if you wait for a marathon, whereas a 5k on a track could be done at any track at any time. There's no scheduling excuse or reason to delay.
Plus, the optics of refusing to participate in an easy training run to raise $10,000 for his favorite charity would be terrible. Even some of his friends would change sides when he refuses to participate.
He won't do it, of course. He'll claim an injury or a recently diagnosed medical "condition," but you can make that statement as part of your challenge.