No pre warmup needed unless you're riding a short TT prologue where every second counts. Pre-stretching is counterproductive, especially on cold muscles. Use the first few miles of riding as your warmup. Let your legs, lungs and heart tell you when to increase the intensity. BTW it likely will be different for every ride. Some you'll feel like hammering from the start and others will suggest that you turn around and park the two wheeled puppy for some couch time If you insist on stretching do some on the bike while riding and/or afterwards but always after the muscles are warm and the blood flowing