For me it's just been about miles. It doesn't matter the speed, the intensity, or the elevation. Doing more miles makes it easier to do even more miles. I've done at least 200 miles per week the last 6 weeks, an amount I would have thought impossible even 2-3 months ago. Just keep riding. Eventually, 75 miles won't seem like much at all. The length of my rides are limited purely by time constraints, not physical ones.
Oh, and don't forget the magical power contained in candy bars. I don't get into the super focused nutrition balancing stuff-- I basically ride more to eat more-- but 50 miles into a ride, a candy bar is like a magical elixir.
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