As a beginner, myself, I find that I do between 60-110 rpm. If you're doing less than 60, that means you're straight chillen AND on flat ground. At your age/weight, I would suggest bumping up the effort to progress or create strength/endurance. I tend to ride only climbs, and I push while descending in order to fast-track my progression; I'm 37yo, for reference- late to the game.