Five intervals of five minutes are a lot for a beginner. Find a flat road and do some 30 second to one minute intervals for 30 minutes. Or, do some ascending intervals, i.e. start with 30 seconds, build up to 2 or 3 minutes in 15 second increments. Take a three minute break in between. You can do these kinds of things in the middle of a regular ride. It works, but I wouldn't do this more than 1 or 2X a week.
I find the kind of hill riding I do serves as intervals. That, and occasionally riding with faster people help me improve. Once a week, I test myself on my own 11 mile time trial course.