I'm listening to Pandora a lot lately, funny how your tastes are your constant even when switching up channels, mentally, musically or philosophically. The music that powers the time on the bike isn't the engine, it's the oil.
Riding the long miles in the sun brings on less fatigue than the shorter winter rides in the snow. The sun adds power, flattens hills, fills muscles, another day over 6 hours is easy, 2 hours in 15 degrees? Hard.