Omnium in this context refers to a multi-race cycling competition, not just a single race. The cyclist trained hard for the omnium, knowing it included multiple events.
Pro Tip 2/3
Sports Usage
'Omnium' is specifically used in track cycling, and using it in another sport may confuse listeners. The commentator excitedly announced the start of the omnium.
Pro Tip 3/3
Olympic Term
Omnium is commonly heard during the Olympics, so associate it with major sporting events. Viewers worldwide watched the Olympic omnium with great interest.