Second wind, spirited ride, into the groove, hard effort, settle into pace, little longer warm up, be at full tilt are all individual descriptions for individual speeds. If heart rate was considered, that number would also vary.

As digibud wisely suggested, you have to listen to your body, but even then, different days will give you different inputs.

I often leave my house a little later than planned, so in reply to your question of How long to warm up?.....

Yesterday's start of the group ride was at 0830 at a distance of 5 miles from my house. I was running late and got out of my garage at 0814. 20 seconds later I was riding at 19.8mph and increased it from there except for the stop signs. 14.03 minutes with 5.01 miles at a 21.4mph average I was at the start of the ride. So in this case the 20 seconds had my juices flowing fairly well.