I think warming up and stretching are separate activities with different goals. I stretch throughout the day, beginning first thing in the morning with the goal of maintaining range of motion. I've found this is increasingly important for me as I get older. Warming up muscles also increases range of motion but in a different way. It's more like an engine coming up to operating temperature. On the bike I spin slowly and easily, increasing cadence and effort in whatever way feels right for that day for 10 minutes or so.