I think Latebloomer might be right about the warming up helping. When I start out too fast it sometimes feels like there is something in my throat clogging my airway. If I take the time to warm up I only get that clogging feeling if I am REALLY pushing it and breathing very hard. When I asked my doctor about this she said I may be experiencing be something called "Vocal cord spasms" and gave me an inhaler to use before a ride. The inhaler only seemed to push my heart rate so high that I can't work hard enough to cause the problem so I think I will stick with a warm-up.