Intervals are a great way to build speed and they are even fun if you like "good pain" but they can offer a good dose of "bad pain" if not ran the correct way. Today I took to my normal route and included a few repeats of .5 miles at 6:00/mile pace. The recovery took up about 1/2 the distance in almost the same amount of time. That is pretty typical of an interval and a good way to keep your HR up, and the "bad pain" down.
1) Avoid speed on tight hamstrings
One of the most important things to watch out for would be any tightness or inflexibility in the hamstrings that you are not able to loosen up with your warm up run of at least .5 to 2 miles. In my experience, adding speed on top of a sore hamstring is a recipe for at least 1 week of recovery. Avoid this scenario.
2) Run from a good base
Another topic to keep in mind are your base miles. You should have at least 4 weeks of consistent training of 3 days/week before doing any speed work. Some sources say more while others will say less however 4 weeks with three workouts per week should be fine as long as you don't put on too much speed... that is the third thing to watch out for.
3) No need for too much speed
Too much speed too soon will lead to those tight hamstrings I was talking about. Once those set in, you will need to back off the speed training and sometimes training all together. That sucks.