> am getting really frustrated trying to keep my heartrate between 150-165, the recommended zone for my marathon training based on age, resting heart rate, max, etc.
You mention “max” - have you determined your real max heart rate? Over and over, people that report difficulty staying within a “training zone” find that their max heart rate is higher than the number they are using to compute the zone.
Lastly - This is an answer I found on the net - not specifically to your question but supports the point that your maximum may not be formulaic, but that may not be a problem - YMMV:
Q I’m in my mid-40s, push myself quite hard in training, and feel great and in good shape. However, after buying a heart rate monitor, I was shocked to find that my resting heart rate was 46, and that I could reach a maximum of 202bpm during hard efforts. Is this safe, or am I pushing myself too hard and likely to go out with a bang?
A Far from being a cause for distress, your impressive heart rate range is something to boast about! A resting rate of 46 indicates that your heart is well conditioned and efficient, with an excellent stroke volume (volume of blood pumped per beat). A useful analogy to help you understand what this means for your body is to imagine using a bucket to bail out a leaky boat. The bigger the bucket, the fewer times you must fill and tip out the bucket in order to keep the leak from sinking the boat. Your low resting heart rate indicates that you have a big bucket. The fact that your stroke volume is relatively high, and your maximum heart rate is over 200bpm is a good thing, because it means that you have a large cardiac output (flow of blood in litres per minute = heart rate x stroke volume). Cardiac output is argued to be the main limiting factor to your maximal oxygen uptake, and the more oxygen you can process, the more efficiently you use energy – an obvious benefit for endurance performance. To put your mind at ease, some elite cyclists have resting heart rates as low as 36bpm, and it is not unusual to see heart rates of over 200bpm. However, your 202bpm is impressive for someone in their 40s, because maximum heart rate declines as we get older. The rough rule of thumb for estimating your ‘age-predicted’ maximum heart rate is to subtract 0.8 times your age from 214 for men, or subtract 0.9 times your age from 209 for women. The fact that your maximum is higher than the number that this equation predicts is not a cause for concern. Maximum heart rates are not as fixed as is sometimes believed, and it is also thought that the decline in maximum heart rate is less marked in those who are physically active. So far from ‘going out with a bang’, it looks as if you’ll be able to enjoy a high level of performance for many years to come. —Dr Alison McConnell, sports and exercise physiologist