1. A method of generating a customized exercise video for a user, the method being implemented by (i) a computer system comprising one or more processors and (ii) a wearable sensor device comprising one or more sensors, the method comprising: (a) identifying a plurality of components, each component corresponding to a respective physical exercise and associated with one of a plurality of categories; (b) assigning a difficulty rank to each component, wherein assigning the difficulty rank to an instant component includes: receiving user ranks for a plurality of prior users that have performed the instant component, receiving user feedback from the plurality of prior users about performing the instant component, and setting the difficulty rank of the instant component based on the user feedback from the plurality of prior users and the user ranks for the plurality of prior users; (c) determining, by the computer system and for each category in the plurality of categories, a category user rank for the user; (d) identifying, by the computer system and for each of the plurality of categories, a respective set of components in the plurality of components having a difficulty rank within a specified range of the category user rank as being available; (e) generating, by the computer system, the customized exercise video including an exercise session composed of a plurality of available components identified in (d); (f) displaying the customized exercise video to the user; (g) receiving, by the computer system, sensor data of the user from the wearable sensor device, wherein the sensor data of the user are generated by the one or more sensors of the wearable sensor device when the user is wearing the wearable sensor device, and wherein the sensor data of the user indicate the user's performance of, or the user's physiological condition during, a physical exercise of the customized exercise video; and (h) modifying, during playback of the customized exercise video and by the computer system, the content of the customized exercise video based on the sensor data of the user by performing an action selected from the group consisting of: (a) changing an amount of a physical exercise of a particular component of the plurality of available components, (b) adding to the customized exercise video a new component having a difficulty rank different from that of the particular component of the plurality of available components, (c) adding to the customized exercise video a new coaching audio clip not initially included in the customized exercise video, and (d) any combination thereof.