Legs say yes, heart says no!

Just pace yourself based on your HR. As you say, your cardio endurance isn't great yet, so just keep working on it. Fitness doesn't happen overnight.
Has your doctor evaluated you within the past 6-12 months to rule out any other problems?

If you're using the heart rate monitor on handles of the machine, take it with a big grain of salt. Many have no relation to reality. If you are puffing and panting, slow down. Otherwise, just keep moving at a pace that allows you to carry on a conversation or sing alone with the music.

Aerobic Deficiency Syndrome. There have been cases of people heavily into crossfit and they can't even walk on a treadmill without their heart rate going through the roof. If you have been doing a lot of heavy lifting and/or "HIIT" or just off the couch or overweight, you really need to work to keep your HR down. Monitor you HR, pick a threshold and stay under it, even if you have to walk slowly.

Are you having any other symptoms at all, besides increased heart rate? Faintness, gasping, wooziness, pain anywhere, sense that your heart is fluttering wildly all of a sudden (vs. going up to the high value a bit more gradually)?

I mean, exertion makes our heart rate go up, and being unfit or doing something unaccustomed makes it go up even more so. If we exert ourselves more, it goes up higher. That's normal. But if there are other bad symptoms, that's a worry, and you should definitely back off and consult your doctor.

It's generally best to start with a challenging but moderate, sustainable pace, and gradually work your way up to including higher intensities. This can help avoid injury, burnout, or excess fatigue.

But if you're simply worried that you're getting above some magical "fat burning zone", don't be: That's pretty much a myth, as a concern for the average exerciser whose main goal is general fitness or increased weight loss.

Also, those "220 - age" formulas for max heart rate are inaccurate for many people. (The formula would put my max at 158, but it's actually 180.

I'm moderately fit (very active for last 15+ years). My heart rate sometimes goes up to mid-160s during exercise, but there are no scary symptoms with it, so I don't worry about it as long as the exercise was intense enough to explain it. (It doesn't go up to 190 because my actual tested max heart rate is 180 - I'm a l'il ol' lady. )

But if you are worried, see your doctor!

Personally, I don't want to be a statistic. (over weight woman who overdid it and died while exercising). So I wear a fitbit with a heart rate monitor. I'm unfit enough that my heart rate gets to 188 doing things that more fit people wouldn't even consider a warm up. I don't want to die, so I pause and take a break, a drink of water, let it slow down, then proceed. I'm doing a light weight and body weight 20 minute routine, then a 20 minute kickboxing routine. Which is fun, but that's where I can overdo it, punching and kicking at imaginary things. Plus there's some jumping in there with a 180 degree switch, that I'm not fit enough (or light enough) to accomplish, so I do a jumping jack or 3 instead, or not. I guess my workouts allow for more variance in form or exercise then a machine.

As you get more fit your heart rate won't spike as much, it will actually get harder to get it up there.

See your doc

Focus on maintaining an easy rhythym in your cardio range for building endurance.

You should periodically push your heart rate to strengthen your heart.

But most importantly....STOP IMMEDIATELY if you have any of the warning signs : dizzy, etc.

Hi! I have a similar issue. I've been running (jogging) for 10 years or so. I'm 48 and at 5'6" I weigh 117lbs. My resting HR is 54, but in longer runs (5 - 10 miles) even when I plod along at 12 - 13 min miling my HR goes into my overreaching zone (147 plus). I can't go much faster anyway! It's really annoying as my perceived effort is fine: I can hold a conversation etc. HR goes back to 100s after a few mins. Currently training for 1st half marathon. All posts above really interesting, but is there anyone out there in my situation, ie. not over weight and with good resting HR but the same prob?

Thanks, that makes sense looking back over my stats.

I am fairly fit, averaging about 40 mpw running and another 15-20 mpw walking for the past two years. I've been a runner for 6 years and was a dedicated walker/hiker for 25 years before that. I am 61 and at a good weight (5'6, 121 lbs).

My resting HR averages about 60, so is a bit high for an athlete. When I start my runs I immediately jump to 145 or so during my warm-up. Even just walking slowly my HR is at 125 or so. Once I am running at an easy conversational pace, I'm usually around 160. I think my maximum is around 185, though I'm not sure. I did an interval workout recently and maxed out at 183 and have been around 184 on hills but I've never tested all out or worn a HRM during a 5k race so I can't be sure that actually is the maximum. I have a wrist HRM monitor and it has occasionally had me at 190 and above, but they are notoriously inaccurate.

In any case, I do have low thryoid issues. I've never heard that that affects HR before. Since I rarely have issues with dizziness etc. I don't worry about the fact that my HR isn't average. I do pay attention in the summer though, because I know heat does affect me very badly. I'll sometimes just sit in the shade to bring my HR down rather than risk passing out mid-run.