Introduction: Carbohydrate (CHO) availability sustains high metabolic demands during prolonged exercise. The adequacy of current CHO intake recommendations, 30-90 g•hr^-1 dependent on CHO mixture and tolerability, to support elite marathon performance is unclear. Purpose: We sought to scrutinize the current upper limit recommendation for exogenous CHO intake to support modeled sub-2-hour marathon (S2M) attempts across elite male and female runners. Methods: Male and female runners (n = 120 each) were modeled from published literature with reference characteristics necessary to complete a S2M (e.g., body mass and running economy). Completion of a S2M was considered across a range of respiratory exchange rates, with maximal starting skeletal muscle and liver glycogen content predicted for elite male and female runners. Results: Modeled exogenous CHO bioavailability needed for male and female runners were 93 ± 26 and 108 ± 22 g•h^-1, respectively (p < 0.0001, d = 0.61). Without exogenous CHO, males were modeled to deplete glycogen in 84 ± 7 minutes, females in 71 ± 5 minutes (p < 0.0001, d = 2.21) despite higher estimated CHO oxidation rates in males (5.1 ± 0.5 g•h^-1) than females (4.4 ± 0.5 g•h^-1; p < 0.0001, d = 1.47). Conclusion: Exogenous CHO intakes < 90 g•h^-1 are insufficient for 65% of modeled runners attempting a S2M. Current recommendations to support marathon performance appear inadequate for elite marathon runners but may be more suitable for male runners in pursuit of a S2M (56 of 120) than female runners (28 of 120).