The same can be done somewhat the same without a heart monitor by not riding with the feeling of cranking, or pedaling with effort. I do this for touring all the time and shift down when I get the feeling I'm working too hard. I also have the luck to have a Brooks sprung saddle that begins to squeak whenever I pedal too fast without resistance (bounce) so I know when to shift up
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As for training for a 50 mile ride: I think all these things involve a bit of breaking through a psychological barrier more than a physical one so the most important thing to do is adopt some basic good habits and just keep pushing your distance and don't get side tracked by "things" until they really become necessary. A bike that fits, good seat, hydration, eating, slowly upgrading my riding kit and a pace that I keep working on improving incrementally is my basic strategy.