I'll just drop the hot take and my 2 cents and take my lumps... Training by HR is unreliable and at best gives only a moderate chance of improvement. Too many variables go into your HR that are not the work you are performing (illness, dehydration, heat, cold, hangover, emotional state...), whereas a PM measures ONLY your actual power output and doesn't care what your HR is or what your PE is. A watt is a watt, whether you felt strong or you felt like you were pedaling through wet cement.

I'm 47 years old, and I am fitter than I ever was back when I raced crits in the late 80s/early 90s, for a variety of reasons, including that I now ride year around, where I most definitely did not do that growing up in western Michigan. The reason why I am fitter, IMHO, was adding a PM to my road bike in 2016, and by 2017 I had one on my gravel rig and 2018 on my MTB. Every ride (commutes too) is a training opportunity. Wait, you say, doesn't that take all the enjoyment out of it? Not at all, I now ride with purpose, while I still get to "learn the contours of the land," and now I actually get to be faster even if "it never gets easier" (except for the recovery rides). I always ride with a PM so that I don't over-train, and I can also evaluate if I undershot my targets. My training program is structured so that I do shorter interval work during the week (maximizing my commutes one way, with a short recovery ride the other), and longer endurance focused rides on the weekends. I still do junk miles with my kids to the local MTB tracks, and I give myself "free days" to go crush gravel. I get to do all the rides I want, but they all have meaning and purpose toward my bigger goals.

Okay so TP premium...don't do a trial until you have the PM and you've ridden with it for a while. It will waste the two weeks. When you first get a PM you're going to see your power go all over the place, it'll be confusing, it won't make sense...for a few weeks, then it will start to make sense as you get a better feel for what different power numbers feel like, and you also get smoother on the crank. If you don't have one, you may want to consider a Garmin Edge that has FirstBeat metrics incorporated to the software (Edge 530, 820 Plus, 1030/1030 Plus). With those devices, and the settings arranged so, a ride of sufficient duration will return estimated FTP and VO2Max values, they can also be used to perform a structured FTP test and return a more accurate value also. A smoother pedal stroke, and some sense of your actual power thresholds will then allow you to get the most out of a balanced, structured training program with the goals you desire, and the analytics in TrainingPeaks will allow you maximize your efforts. Having a coach is helpful, but not required, there are several great books that you can get to help you understand the physiological

Apologies for the length. Bottomline: TrainingPeaks can be a great tool, with a little forethought and preparation, you can really get a lot out of it, in my opinion. I also am not an expert or a coach, but I've been training with power for over 4 years now, so I have some layman's experience.