Heart rate

I think it depends on a lot of factors, such as your age, target heart rate, what your heart rate reading was while on the treadmill, whether you have any medical conditions, etc.

You may find this information below to be helpful. I am no medical expert or anything like that, though. I also definitely go over 85% of my "max" heart rate when working out, mostly when I do interval workouts or when I am playing hockey, but I don't typically maintain that level for extended periods of time. On the site below, maximum heart rate is only an estimate based on your age; it can vary based on other factors as well (that's why I put max in quotes above). I hope this helps.

http://my.clevelandclinic.org/heart/prevention/exercise/pulsethr.aspx

What is target heart rate?

You gain the most benefits and lessen the risks when you exercise in your target heart rate zone. Usually this is when your exercise heart rate (pulse) is 60 to 80 percent of your maximum heart rate. In some cases, your health care provider may decrease your target heart rate zone to begin with 50 percent.

It is not recommended to exercise above 85 percent of your maximum heart rate. Intensity at that level increases both cardiovascular and orthopedic risk with minimal, additional health-related benefit from the exercise.

Always check with your health care provider before starting an exercise program. Your health care provider can help you find a program and target heart rate zone that matches your needs, goals and physical condition.

When beginning an exercise program, you may need to gradually build up to a level that is within your target heart rate zone, especially if you have not exercised regularly before. If the exercise feels too hard, slow down. You will reduce your risk of injury and enjoy the exercise more if you don't try to over-do it!

To find out if your are exercising in your target zone (between 60 and 80 percent of your maximum heart rate), stop exercising and check your 10-second pulse. If your pulse is below your target zone, increase your rate of exercise. If your pulse is above your target zone, decrease your rate of exercise.

NO she is wrong. Healthy athletes regularly keep their heart rate in the high end of the aerobic zone for 4+ hours

If she could see your heart rate displayed she might well get worried as maximum heart rate (as well as various zones) decrease with age. When I train if I'm hitting 172 bpm I'm pushing like crazy up in zone 6, my 14 year old at the same heart rate is comfortably in zone 4 and could keep going for hours.

Need more info but unless you've got medical problems I'd say you were pretty safe, if you could run for 30 mins then your heart rate is unlikely to be up above zone 4 for long (unless you're super fit).

Was she thinking you were running fast enough to be in ANaerobic state?

Aerobic is using oxygen -- exercise that improves the efficiency of the body's cardiovascular system in absorbing and transporting oxygen.

ANaerobic is NOT using oxygen -- exercise that does not improve or is not intended to improve the efficiency of the body's cardiovascular system... (this is what you are using that last 1/8 - 1/4 mile when you are sprinting to the finish. It is okay to incorporate anaerobic bursts into your aerobic activity but most people do not and cannot sustain anaerobic activity for very long.

Unless you have some illness or disease that has rendered your heart unhealthy, have been told by a doctor you may not exercise...you should be fine. I doubt you are running at > 85% of your max heart rate for 30 minutes....it is not sustainable unless you are a top performing athlete.