I am not an expert on interval training, but I do know that your body will
get used to any set routine.  My trainer has given me an eight week routine
that starts:
Day 1: 30 minutes - heart rate moderately, evenly elevated (for me, at my
age that's about 145-155)
Day 2: 3 sets of 3 minutes at threshold, 3 minutes rest.  Threshold goal for
me is about 160.
Date 3:  rest
Day 4:  20 minutes - easy (I try for heart rate at about  130-140)
Day 5: 15 minutes all out, threshold.(no comment).
 Of course, cooldowns are tacked on at the end.  Warms ups at the start.

It goes on from there...week two increases to 35 minutes cardio, 1 more set
of the interval, the threshold interval increases in time, etc.  It
eventually goes up to 50 minutes and weird things like a fartlek( I think
that's what it said...can't remember and my gym bag is in the car!) which I
have no idea what that is (never got that far before my enforced non cardio
period).  The long day also dips in the middle back to 30 minutes for one
week.

Anyhoo, my point is that there are a zillion plans out there and you are
probably in better cardio shape than me and can probably start at a
different place, or pace.  Ask your trainer for a cardio program and I am
sure she can come up with one for you that varies the intensity for you.
I'd be happy to send you the whole 8 week deal of mine, if you'd like.

Also, I was finally literally shoved onto the Stairmaster, and while I truly
find it a modern form of torture, I have to admit it gives a wonderful,
sweat producing, fat burning workout.  Try that for 15 minutes on one of
your 30 minute days and skip to something else really quickly while your
heart rate is still up.  Oh yes, surprisingly, with my pre-surgery lousy
knees, it didn't hurt!  I always stayed away from it for that reason, but
there is something about the way the steps move with you that make it easier
to do  than to  go up real steps.(IMO).  I can't remember if knees are an
issue with you!

Good luck!  Elise.