Could I do high intensity intervals when I feel up to it?

If you are looking to optimize cardiovascular conditioning, you can do high intensity intervals.  If you are seeking wellness, we generally recommend focusing on getting your intensity up to a consistent 70% to 80% of your theoretical max heart rate for the duration of the session.

You can calculate the theoretical max heart rate by taking the number 220 and subtracting your age.  Then multiply that number by 70% or 80% and those numbers would be the target range.