I take it easy and build up after 30 mins or 6 miles etc.
I usually stretch on non riding days to keep supple/flexi.
Also try a warm/hot shower before you ride but make sure you are super dry before going out or will freeze.
Wear your kit in your house for 20 mins and you'll add warmth to your tissues.
SFB is right stretching cold tissues can cause tears, limits power for nearly 30mins and should be done after the warm up.
Gentle stretches after a ride sure.
You can condition the body based on warm up rides, stretch on non ride days and stretching before an event can sap power according the american journal sports medicine 2009.