As in a herd of cattle, a pride of lions, or a murder of crows, a "marathon" is the collective for meetings, as in "a marathon of meetings".
Between putting this definition into Urban Dictionary and my marathon of meetings, don't expect me to get any actual work done today.
When a person with almost no training runs a full marathon. The first to document this run was Bert Kreischer, who has what's know among scientists as the Mickey Mantle Gene.
Jake woke up today and decided to run a Mickey Mantle Marathon. Those 26miles almost killed him.