I would guess for the majority of us " fitness" actually means good health rather than the development of muscle and muscle stamina. Good health does of course require exercise but equally important are diet and lifestyle. Professional athletes invariably have professional support to ensure all of those elements are monitored and maximised but it worries me a little that non pros merely relate effort expended to potential fitness increase when in reality they could be doing more damage than good to their health. I think it has been shown that the average age of EMTB owners is somewhat greater than the average age of analogue mtb riders. There comes a time/age when low intensity cardio vascular exercise is far more beneficial and less harmfull than high intensity exercise, and if your aim is to increase muscle and stamina I'm afraid you will have largely missed the bus physically anyway