"Epic" had been used by skiers for to describe powder days for years...

As for an "epic" mountain bike ride.... its' a standard description for a long mileage singletrack ride that involves at least two of the following: long mileage, bonking, at least one mechanical, running out of water,Running out of food, injury, unexpected weather change, extensive hike a bike,getting lost...

Unabriged Dictionary:
1. noting or pertaining to a long poetic composition, usually centered upon a hero, in which a series of great achievements or events is narrated in elevated style: Homer's Iliad is an epic poem.
2. resembling or suggesting such poetry: an epic novel on the founding of the country.
3. heroic; majestic; impressively great: the epic events of the war.
4. of unusually great size or extent: a crime wave of epic proportions.
–noun
5. an epic poem.
6. epic poetry.
7. any composition resembling an epic.
8. something worthy to form the subject of an epic: The defense of the Alamo is an American epic.
Urban Dictionary:
. Epic

(adjective) Awesome, kickass, or otherwise positive. Can be used to refer to anything but is usually referring to a particular event or action. The most common usages are "epic win" or "epic failure," and some prefer to type it in all caps. Occasionally people use the phrase "Epic ___" as a stand-alone sentence or phrase, always following a story about something considered Epic.