Whenever you make a huge leap in your training, and it can seem a little intimidating. You should be excited though, because you are ready for this! You've been manipulating your interval structure based on heart rate recovery. You are avoiding unnecessary spikes in your heart rate that could force a systems-wide shut-down. Instead, you have been training for gentle undulations that keep you right in the middle of the spectrum while avoiding the extremes. This is all based on work-to-rest ratios, and using this concept you can successfully switch from structured intervals to continuous aerobic activity.
The structured intervals use specific ratios to allow for adequate recovery. Now you can tie everything together, and keep you right in the "sweet spot" of your training zones. You'll want to simply run by feel for now. This will allow you to notice when your heart rate starts to get too high and your breathing becomes heavy and labored. These are signs you are too far "out of range" and need to slow your pace or take a walking break to get back into the "sweet spot." Your goal is to spend the largest portion of your workout in that zone, with your heart rate elevated but not shooting out of control.
This isn't going to happen magically; the first few times will be a learning experience. How fast is too fast? How high is too high? How long is too long? Since everyone has different levels of fitness, there is no one-size-fits-all solution. You'll want to use these cues to help regulate your individualized intervals:
Take your foot off the gas pedal. The goal is a longer, sustained effort at an easy/moderate pace. The more relaxed and comfortable you can stay, the better off you'll be in the long run.
Listen to your breathing. When your breath becomes audible, it is a good indication your heart rate is getting too high and you need to ease up a bit.
Walk it off as little or as often as necessary. You will still benefit from the workout, whether you take ten breaks of 30 seconds apiece or five breaks of 60 seconds.
Don't judge yourself. Each workout is going to be a bit different, and this is the first time you'll be exercising control. Stay loose, stay relaxed. Listen to your body, and respond accordingly.