Although I keep track of the machine readouts, I don't depend on them to judge my workouts. With each type of cardio, I have a set goal; examples:
- brisk: work at my aerobic capacity, just short of being out of breath, and increase my endurance at these speeds
- high intensity: work above my aerobic capacity into my lactic acid threshold or VO2 maximum; I do get out of breath and feel like I will burst. The goals are to force my cardio vascular system to deal with the stress, thereby becoming stronger, deal more efficiently with lactic acid, and increase my jogging/running intervals.
I increase the time/intensity and do it until it becomes more comfortable, before increasing again. The machine readouts just reinforce my progress, because they go up as I do.
Do or don't do; there is no try (Yoda)