There are widely varying stances on optimal exercise heart rate, but there is some common ground that most people agree on.
- If you perform any kind of activity at maximal (or close to it) heart rates, your exercise duration will necessarily be brief, as your body simply can't store and use enough glycogen to support this kind of effort for long durations. While you can increase your ability to store and use glycogen, there is a hard upper limit on how long you can go at these types of intensities.
- If you work out at too low an intensity, you don't really do a lot with regard to increasing fitness or burning calories unless you work out for a long duration, which if you're just beginning may not even be within your capabilities for a while.
Hence, the suggestion of intervals is a good one. This allows you to burn and recover (somewhat) in turns and increase your workout duration beyond what you could do if you're close to your maximal heart rate all the time.
Alternatively, there is the notion of base building, where you just slow down until you're exercising in a mid-range heart rate and go longer. Many athletes perform this type of exercise in the "off season". The idea being that you're performing at a level where your body can still acquire some of its energy by converting and burning fat, but high enough that you're training your body to work a bit harder. The goal of this type of exercise, then, is to increase the duration for which you can perform at a given heart rate (say 60-75 percent of max) and also increase the speed at which you can perform at that same heart rate. Then, once you've built up a larger aerobic base capability, you can add intervals, hills, speed work, etc. to add power and speed to your ability to "go long".
A good estimate for the sort of base building heart rate you may want (give or take a bit) is 180 - your age. Staying within 5 or 10 bpm of this will provide a bit of flexibility, especially when starting out as it's hard to measure effort.
Another thing to note is that this requires a lot of patience. If you're just starting out and running for 8-10 minutes ramps you immediately into 190-200 bpm, you may find that to keep your heart rate in base building range you have to go at not much more than a slow shuffle, or walk 100 yards and run 50, or some such. This can be pretty frustrating, but in my experience has been well worth it.