Once a person has established a habit of running, it becomes an addiction much like other addictions. Soon, you feel like you can't live if you don't go for another run (maybe I'm exaggerating slightly...but not much). This is often refered to as the "runners high." I won't take the time to explain the science behind the "runners high" (mostly because I don't completely understand it either), but suffice it to say this is the point at which your running really has the potential to improve drastically; you now enjoy doing it, your level of fitness has increased, you are getting faster (and probably a little bit lighter). However, with the new found desire to run comes the need for responsibility. If you go from jogging a half hour, 3 times per week, to running 6 days with 3 speed sessions, you are going to injure yourself. An injury or an illness is the last thing a runner wants during training. so, in order to prevent this, run responsibly!
Many experts have suggested the "Ten percent rule" which says your weekly mileage should not increase more than 10 percent from week to week So, if you ran 20 miles last week, according to this rule you should not run more than 22 miles next week. Now, depending on what you are training for, what kind of fitness you are in, and many other factors, this might be a rule you can break, but this is a good rule to keep in mind. Either way, the idea is to slowly increase your total mileage, the distance of your long run, and your speed work.