Quick questions on interval

by Brian
(Gainesville fl)

Thank you for such a great site first off, it has helped me learn and educate my friends how to interval train.. I'm 18 and would like to know where I should start. I run 5 times a week around 7 mph for around 2 to 3 miles.
I'm 5'5 and 138 right now
I'm trying to loose that chest fat that has accumulated around my pecs.
I also have a small gut that I would like to eliminate.
I tried the 10 reps today and found it difficult but a good work out
I have to admit I had to do 7 straight and then stop and walk and then did 4 in a row.
That's 11 but I did it because I felt bad that I stopped.

And also can you walk instead if jog after you are at a 10 intensity or would you recomend to stay at intensity 6 for the 30 seconds...

Hope you can help thanks

Brian