I don't track heart rate or RPMs or watts on my rides other than indoors on the Wahoo trainer. But I do know that getting your heart rate consistently down for a particular amount of effort will take a good deal of miles on the bike, i.e., getting in shape, optimizing as well as you can your fitness, cardio, etc. As others have said, just getting out and riding as much as you can for now is best. I find that riding multiple days in a row, usually three but more if I have the time, gets me in the best shape (which I mostly gauge from not getting out of breath on climbs). Those don't all have to be big effort days but the repetition is key.
Also key is for target heart rate is what your resting BPM might be. And that seems to really vary among cyclists, whether in great physical shape or not.