+1 to good, well-fitting shoes - a necessity to avoid injury - and good running clothes - which make it SO much more pleasant.

I don't know about doing intervals more than once per week though. Your body needs time to recover from such a hard workout. Plus, if you only do intervals, you won't ever learn to run at a comfortable pace. Intervals are a great way to increase your speed and cardio fitness, but they're best as part of a program that includes other types of run. And if you just want to run without being so concerned about "training" for something or other, do it and forget the interval days!

As far as what distance you "should" be running, let your body tell you. Long and hard enough to challenge yourself, not so long or hard that you're excessively sore the next day. Your interval days will be your shorter runs, speedplay and tempo runs a bit longer, then you can have "long run" days which is whatever's long for YOU. For me a "long run" is 5-1/2 or 6 miles, for some people that's practically a sprint distance. For you right now it could be 2 or 3 miles. There's no "should!"