Hi WC.
Please please please don't use that outdated formula. It's meant to tell you your max. HR but most people find that it is actually miles off. This can lead to people training in the wrong HR zones which can be dangerous. In short, it is not fit for purpose.
Instead, find your own resting HR by measuring your pulse before you get out of bed for a few mornings then calculate the mean average.
To find your max. HR, find a time when you can give it your all on an interval session. Build up the intensity for each repeat. For you final repeat, push to your absolute limit and try to hold the distance for longer than your other repeats. This should give you your approximate max. HR (some people add on 5 bpm as it's so hard to actually achieve max.).
I'm assuming you have a Garmin watch or similar? Enter the two values into the watch, then when you're out running, cycle through the screens until you see the HR screen. This will tell you which HR zone you are running in at any given time. 80% of your weekly miles should be in the aerobic zone (this is an easy, conversational pace).
