Heart rate guidance is too complex to give online by the likes of me. Some may wish to give it a go, but there are too many ways to get it wrong for me.
You are only 3 years my senior, but you are 40 - 50 pound heavier which carries varying risks dependant on the weight, but even if it's pure lean mass there could be risks.
If you are finding the same routines are losing their impact I would say time for a change. There are a number of different styles of interval sessions, my preferred is Timmons but when that stops working I will switch it for another.
I extend Timmons to being 10 sets instead of 3 because I only do one of them a week, but it has helped me. It is supposed to operate on VO2 max % but I do simple flat out sprint for 20 seconds then 2 minutes steady, which is helping increase my average pace, even while my weight is climbing.
If you are looking to lose weight and aren't being careful about diet, it will fluctuate and slow the lower your body weight becomes, because there is less of you to carry so you burn less fuel in training. This is in part countered if you are gaining lean mass, by the energy this burns all the time, but it's not easy to calculate.
Training for fat loss needs variation, so weights, intervals and some static state or other cardio will be an ideal mix. Make sure you change things around a bit every few months or so to keep the body on its toes.
I know this doesn't answer your question and apologise for that but the heart is a risky measure to advise on when not face to face. My bpm on distance run is close to 180, approx age related max, and I am comfortable. One of my colleagues who is just over half my age would likely die if his heart rate went that high for more than a minute, he is somewhat spherical.
I hope what I have said is of some use. If not do let me know and I'll see if I can do better.