Having had the opportunity to compete in single day, stage, 12 hour, 24 hour and ultra racing in both road and mountain bike I have been able to train and adapt for the different disciplines, helping me to evolve as a cyclist and shaping my ability to judge effort without the use of power and heart rate measuring devices.
In 2018 I competed in BIKINGMAN OMAN ultra, my goal - to finish the 1000km sprint race in less than 3 days, to achieve it with a minimalist approach by carrying only what was needed and ride without power and heart rate metrics to extend the battery life on the Garmin and not get caught up in the numbers. I opted to ride with only distance, time, speed and GPS. By monitoring pressure applied on the pedals, breathing and overall feel I was able to control my pacing effort.
Ultra-endurance cycling is all about doing the ordinary extraordinary well, it is about pacing, burning just the right amount of energy and keep the tank topped up -take advantage of the free speed, spend less time pedaling and decrease energy expenditure.
Through trial and error, and with the use of a RPE scale I have learn't what MY! too slow and too fast feels like, to ride where pedaling feels almost effortless and somewhat fluid, speed is reasonable and breathing rate is easy to rhythmic to briefly deep for ultra distance racing,
RPE is useful in training as well, especially when you don't have a power meter or heart rate monitor to gauge your effort.
RPE 1-2, easy spin or light pressure on the pedals, minimal sensation of leg effort or fatigue. This is known as Zone 1 training where average power is below 55% of FTP and heart rate is below 68% of LTHR.
RPE 2-3, all day effort, breathing is more regular than during an easy ride, but you are still able to hold a continuous conversation. This is known as Zone 2 training where the target average power is between 56%-75% of FTP and heart rate is between 69%-83% of LTHR.
RPE 3-4, a "spirited" group ride, breathing is deeper and more rhythmic than during an endurance ride, conversation may be somewhat challenging, This is known as Zone 3 training where the target average power is between 76%-90% of FTP and heart rate is between 84%-94% of LTHR.
RPE 4-5, race pace, continuous sensation of moderate or even greater leg fatigue, deep and shortness of breathe, difficult to hold a conversation. This is known as Zone 4 training where the target average power is between 91%-105% of FTP and heart rate above 95% of LTHR.
RPE 6-7, vigorous efforts lasting between 3 to 8 minutes with strong to severe sensations of leg effort or fatigue, breathing is ragged and conversation is short. This is known as Zone 5 training where the target average power is between 106%-120% of FTP and heart rate above 100% of LTHR.
RPE >7, very hard and short efforts between 30 seconds to 3 minutes, with severe sensation of leg fatigue, can barely breath and able to speak a few words. This is known as Zone 6 training where the target average power is between 121%-150% of FTP and heart rate is generally not use as a guide.
RPE credit/sourced: Training and Racing with a Power Meter, Hunter Allen and Andrew Coggan
Don't get caught up in the numbers, pay attention to how it feels.