Gym Brick Sessions
You can start in the Gym or stay in it for the full session. You can start on the bike and then go out for a run or just transfer to the treadmill. Do what you feel you need to but don't spend longer than 25 minutes on the gym bike for the sake of your own sanity.
The same applies to the treadmill. I can't handle those things for longer the 15 minutes so only use them for as long as you are comfortable.
Multi Brick
This makes the session a little more interesting. Simply switch between bike and run every 5 minutes. I find that 30 minutes of this is enough. Go longer if you want to but remember that the whole point of these sessions is to get your body used to switching between cycling and running.
The same applies to the treadmill. I can't handle those things for longer the 15 minutes so only use them for as long as you are comfortable.
Multi Brick
This makes the session a little more interesting. Simply switch between bike and run every 5 minutes. I find that 30 minutes of this is enough. Go longer if you want to but remember that the whole point of these sessions is to get your body used to switching between cycling and running.