You’ll note the comment on that workout is “this workout is generally done with the same nutrition, gear, equipment, and apparel expected to be used on race day.”
It’s not a requirement to do it on your road bike, but recommended. The gym option is a great alternative. However, in your plan the brick workouts take place mid week every week, so I expect that if you can’t do the brick this Tuesday, you can’t do it any week-day and you do need to get some time doing a race simulation.
So, you are correct that the next best option is to swap the week-end ride with the brick. This is a great solution.
Thanks David. I actually figured out my brick workout. I brought a lock and locked the bike to a fence, and then I ran where I could mostly keep the bike in my eyesight, so very short laps but it all worked. Also had to keep running shoes in a bag over my shoulder. But wasn’t an issue at all. So I figured the bricks out, yes I did see that it should be same nutrition, gear and equipment so that’s what I did.