Fitness Videos are a training item used to increase the player characters health score. It costs 50 XP and one night training to increase the players Maximum Health by 2-4 points.
Since fitness videos are presented by B and C List British celebrities, and involve them dancing around in form fitting clothing, the videos also have the side effect of increasing current lust by 2-4 points, but this is easily dealth with by masturbating. They are a consumable item - the player character throws the videos onto the fire after the end of a session. The fact that this item is a VHS tape rather than a DVD possibly indicates that the game is set in the recent past - or possibly the island is just behind the times.
The names of the videos are picked randomly from a list of genuine fitness videos starring genuine B and C List British celebrities.