I figured out why I was so slow the other day. I thought it was riding without my cleated shoes and wearing a sports jacket. Noooooo, last night I woke up with my throat on fire, every muscle aching and sneezing and coughing- I was getting a COLD!!
Had a really busy day ahead of me at work today and had to pull myself together after about an hour and a half of sleep. I felt like crap but was in no mood to deal with buses or driving into the city so I got out and rode. Fortunately the temperature was about 50-60°F with strong wind gusts at my back for about 70% of the ride in- the downside of that being that the ride home was pretty rough and really slow.
Riding the 10 mile ride home when I feel that "out of it" is like the end of a tough double century, just keep the legs going around until you get there.