To keep rhythm I use a watch connected to a podometer and a heart-sensor.
I look at the watch once or twice per minute maybe, I don't think about it, it's just a routine.
- if my running rhythm is too high: I can see that the frequency of my heart is slowly increasing, so I can adjust my rhythm and slow down just a little. This can be done long before I attain the max heart frequency where exhaustion-fatigue happens.
- the podometer gives me the speed I have when my heart is in its comfort zone. So every now and then, I try to adjust to this speed.
I don't keep the rhythm, I just follow it, only following these two parameters. It's easy.
To calculate the max frequency, the rule of thumb is 220 - your age.
I am 39, my maximal heart frequency is 220-39 = 181 bpm. And I try to run under 181-18 = 163 bpm. (18 here for 10%, though I have read it could be 20%)
I can run much much much longer since I have been following this rule.
It correspond to the difference between aerobic and anaerobic. When running in anaerobic mode, your body has not enough oxygen, and this is the source of exhaustion and fatigue.
You could find some more details here for describing the difference between running in aerobic or anaerobic mode: https://runneracademy.com/aerobic-vs-anaerobic/
Personnally, the model I use is Polar RC3 GPS with podometer and heart sensor. I guess there are lots of other models, and other trademarks that does that too.
I think there are also watches that only do heart-sensor.
That could be sufficient just to check that you don't go over 10-20% less of your max heart frequency.