Calculating the Limits of Endurance - Intensity versus Time

Thurber et al in Herman Pontzer's lab at Duke used the Race Across USA (what dummies!) to look at metabolic rate relative to basal metabolism. Over an arbitrarily long period, humans can only sustain effort at 2.5x resting metabolism. In a marathon it's almost 16x (for less than 1 day), in the Tour de France it's almost 5x (for 23 days) and for someone crossing Antarctia it's 3.5x for 95 days. Lay article here, paper here.