TIRING.
I started my commute home from a friend's office yesterday, about 5km from my office. Decided to take advantage of a change of pace and took one of the other east-west bike routes home from there, and grab a north-south one closer to my house. I hadn't realized that this decision would bump my total ascent home from ~370m (~1200') to about 550m (~1800'), including a segment of average 11% grade x 0.5km (0.3 miles) that I had to climb about 20km (12.5 miles) into my ride.