I use TrainingPeaks Premium to track my training and training plans to give me direction. TrainingPeaks has many training plans available which plug right into it. I've heard good things about TrainerRoad from riders who post here. Strava is fun, but it's not that interesting from a training perspective. Zwift is certainly all the rage here. Personally I don't need that sort of thing for motivation. My motivation is internal - faster, stronger, better.
On another topic, if you are really getting into training, you should put a power meter on your bike, and you might as well get a 1000 series Garmin to display info, put outdoor routes into, and upload into whatever training app you're using. Money, though. Heart rate monitors are great for recording effort on outdoor rides, but are not that useful for doing prescribed intervals on an indoor trainer.
The standard training book is The Cyclist's Training Bible by Friel.
This is all technical stuff which does offer advantages. However the main thing is simply to ride lots and outdoors to ride hills, lots of them. Gradually increase the length of your efforts and then increase their intensity. On the bike, not much happens in under 45 minutes, so that's the first thing to work on. It is said that distance equals strength. The longer your individual rides, the stronger you get. In my 50s, I went from dead in the water to a double century in 3 years with only a heart rate monitor, no software, and I'm not talented.
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Results matter