Lots of slow long workouts aren't bad as long as you mix in a few speed days. Especially early in a training program slow miles are how you build up your body to the point you can handle speed work. I was just reading an article today that talks about speed training and he lists some speed workouts assuming you can put in a couple miles warmup before starting the "speed" portion of the training, so until you build up your mileage some I wouldn't worry too much about speed. The frequency and type of speed work you are doing now should keep your legs and lungs used to going faster than normal, which is the goal. Doing longer warmup and cooldown would be less beneficial than adding an extra repeat or fartlek within the workout itself. Keep your long days long and your fast days fast, they work the body in different ways.

http://www.birkie.com/?page=1029&trainingid=53