Haven't you ever bonked?

All that energy stored in fat and protein will only let you walk or coast home if your muscle glycogen is depleted. If your event lasts more than a couple hours, glycogen conservation is a major concern.

You minimize glycogen utilization by:

1) Maximizing your running velocity or riding power at VO2 max to allow sub max intensities to be as fast and as 'aerobic' as possible. That will allow the metabolism of lipids to be as high as possible (by keeping your race pace as 'aerobic' as possible). By 'aerobic', I'm not talking simply keeping HR as low as possible or keeping O2 consumption as low as possible. You should be keeping you respiratory quotient as low as possible at any given running speed or cycling power output. Low O2 consumption or HR is of little value if you're exhaling a comparatively large volume of CO2 with every breath (indicating higher glycogen utilization). Higher cadences may increase O2 consumption, but the rider's respiratory quotient should be lower (indicating greater lipid utilization / less glycogen utilization).

2) You minimize the forces your muscles are forced to exert. Smaller, aerobic, slow twitch muscle fibers are recruited first. Faster twitch motor units aren't recruited if the forces required can be handled by the smaller slow twitch fibers. The recruitment of your IIa's & IIb's is minimized with low force demands. Minimize recruitment of faster twitch muscle fibers - minimize glycogen utilization.