I'm no expert, but yes intervals feel like hell. They are something you want to ease into and there are many variations in the types of intervals that can be done. Each type depends upon what you want to improve in. In general, the more intense the interval, the shorter the interval time and visa versa. You should start out by doing some moderate intervals like 3 sets of 1 minute at 85-90% of your max heart rate then 1 to 2 min recovery. As you get used to them, you can increase the number of intervals in the set and the duration of the interval. Be careful with them; they can lead to overtraining. Some coaches don't advise more than 2 interval sessions a week with good recovery time in between. It's best to get a coach of at least a good training book by coaches like Joe Friel and Chris Carmichael to learn about the many types of intervals that can be done and how often to do them. From what you wrote, it seems like you are going too hard and maybe didn't hydrate enough. Were these done indoors? If so you need a good fan and lots of sports drink.