Well, I guess all exercise tips are given with the assumption that the OP won;'t be exceedingly foolish and hurt him-/herself. Short intervals are good for building cardiovascular, lung, and recovery abilities. Longer intervals at varying intensities in various gears build those same things, but focus more on building different muscle groups as well. Longer rides at a higher cadence builds aerobic and CV .... and squats build muscle.
more than one way to get from here to "faster," and usually a combination of ways works.
And if a person so desires, s/he could hurt or even injure him/herself quite badly by exercising wrong. I think we all assume the person is smart enough not to ... otherwise the only really reasonable advice would be "Put on your helmet and crawl back into bed."