I never stretch but I will take it easy for the first mile or so. Then I'm blasting all the way -- usually 13 miles in and 20 miles home, depending on route and circumstances. I'm not in it for a world record but I know that I have to go at a certain, quick pace in order to make it to work in time, accounting for a flat tire.
I liken my commute to a long sprint. The stop lights give me a breather.
I run a lot, too, and cross-training is valuable. I couldn't ride 5 days a week. I'll often not ride on the weekends. Usually, by the end of the week I'm happy to set the bike aside in favor of a long run. I almost always take a weekend day off, normally Sunday.
I drink enough water to fill a lake, I eat all through the day, and always eat a banana or such about an hour prior to ride home. I always have a good-sized bowl of oatmeal before ride in. I have two water cages, and a Gatorade bottle usually is in one of them. I'll consume both each way.