When entering the Devil's Secret Training Area you'll see a green fire rising from the ground. This is the Hell Fire Pit, it is the first training point of the building and requires 1B Body Toughness to train in, and to afk-train you'll need 4B Body Toughness. To start training in it (under the requirement), you'll need 50M Body Toughness. It will multiply your Body Toughness by 2K.
The Hell Fire Pit's appearance is, quite simple. Green mist comes from a drain clouding the area. However, despite this, the Hell Fire Pit is not very long. If you want to explore the rest of the area, but you don't have 1B Body Toughness you can attempt to teleport around it.
To calculate how much Body Toughness you train per minute/hour multiply your Body Toughness multiplier by 2000 then divide by 1.25 (This is the interval in which you get BT) Proceed to multiply your product with the amount of seconds you are going to train for.