On the wind thing, for myself , I know there are days when I take a certain amount of satisfaction in boring a hole through the wind and other days when the wind bores a hole through me. On those off days, pick a steady pace that can be maintained. Often, as muscles warm up everything feels better. On the speed thing, the amount of energy it takes to propel a bike from the middle range of speed to high speed increases expotentially.
The link,
Bicycle Speed And Power Calculator is a mini-program to calculate speed verses power for a variety of cyclists. You can play around with the program and see how power requirements vary under different circumstances. One of the variables is tires. The faster you go the more important smooth road slicks will be.