Being young and in great shape means a lot in terms of having the stamina and strength to hike Whitney, but not much in terms of altitude sickness. Indeed, I seem to recall reading in one report that, statistically, women over 50 have the lowest incidence, followed by men over 50, followed by women under 50, followed by men under 50 (though reports generally state that sex and age and fitness do not correlate with AMS). If true, the differences may be explained by speed of ascent: a huge, but by no means only, variable in terms of risk for AMS, HAPE or HACE is speed of ascent.
The long and short: if being a fit marathoner helps you keep a steady, but not overly fast rate of ascent, it will be a benefit. If being a fit marathoner makes you race up the mountain, it may be a liability.
Good luck, have fun, and be safe.
