Should I use my heartrate monitor exclusively in determining how I ride for the night? Answer...drumroll please...NO!
But, you said go out and buy one and we used it to determine our Threshold. True...however, the heartrate monitor comes with its limitations. Bottomline is you need to incorporate your heart rate, resting perceived exertion and other factors to determine what intensity you will train for on the ride.
Here are some examples with two other factors Resting Heart Rate;RHR & Cycling Power;CP in addition to Training Heartrate;TH to consider when determining what intensity to train at or to answer...what the H$*# is going on here with my legs(Bill B. and Deb W.):
RHR Up: overtrained...rest!
RHR Up/CP Nrml/TH Nrml; Mental Stress/Anxiety...take a yoga class...Relax!
All Normal; Training is going well ;)
RHR Nrml/CP Low/TH Nrml; common during Base Bldng(like now...pushing too hard too soon..Chill!).
RHR Nrml/CP & TH cannot be obtained; you my friend have "dead legs." Take it EZ or take the day off.
RHR Nrml/ CP goes way down and TH is dropping or hard to keep at prescribed zone; most likely dehydrated.
There you have it...as you can see its NOT all about the heartrate monitor. Determination of training intensity is multifaceted. Therefore, become a "smart" rider and listen to your body while at the same time assessing what is coming back from the computer gadgets around you. All of this input will help dictate how you approach your training plan for the night. Ride smart... Cheers!