I think there are so many variables it's hard to say really. My guess is the higher level you are and the nearer it is to the race then the less you want to take this sort of risk and the less beneficial it is. The risk is hurting yourself and missing training days or worse.

Personally I am very middle of the pack so I enjoy this sort of thing. But that is in the context of a not very well thought out training plan. For me the benefits include building confidence and testing nutrition. But I'm also in the position to be able to devote a whole day to it and not be letting anything else slip. I've even done it where I cycle to the lake, swim, cycle back via a longer route then run. The difference is it's all at an easy pace and I drag out the 'transitions' by having a chat with someone or putting a slow cook meal before running so it's ready when I'm back.

With your idea I'm guessing the bike by itself would be a known quantity. So then it would get more interesting on the run. I'd be thinking of doing it only if I had no niggles and ideally with a running route where I could jack it in if I started to feel too sorry for myself. That might be loops never too far from home or a known someone who could pick you up. With loops you can also leave drink bottles to pickup on the way past home. If you do then take the opportunity to practice as many race specific things as you can - nutrition, clothing etc.

I’m a novice, but with the run carrying form breakdown and injury risk....I reverse the brick. Run then bike.

I've split that run and have done a 45' easy run, 2.5 hr bike, 45' run.
That was on a test weekend though, working on pacing and nutrition for a HIM.

In my experience chasing missed workouts is generally a slippery slope the has ended in injury.

But if you want to proceed, I’d suggest something slightly different. Do your long ride in the AM (and skip the brick run if you normally do one). Then do your long run Sat evening. Gives you some time to recover so your running form will be fresher, lessening injury risk.

From time to time I’ve done this, pulled forward my Sun AM run into Sat evening after a long ride Sat AM. Usually because I want a full Sunday with family. I just really make sure to nail my nutrition between workouts, and make sure to recover well the day after.

If it's a one off missed session, you could let it go and just do your bike ride as normal.

An option not mention yet is to run first, so that the legs are fresher say run 1 hour then ride 2 hours. You will likely need to adjust your pace and will need to fuel from the start of the ride. You get in 3 hours if endurance work. If you really wanted you could add a run off the bike.

The reason for running first is to minimize injury risks and maintain run form. Running off the bike has its place, but only with good form, uses its a race where it may get ugly.

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When getting back into competitive running in the early 2000's after a 10-year hiatus, I used the exact protocol that you are considering: a 2-hour bike followed immediately by a 90-minute run. The goal for me was to maximize a singular endurance workout while minimizing the impact (I was prone to lower-leg stress fractures, even at collegiate weight of 160 pounds). The resultant 3.5 hour combined workout boosted my aerobic capacity like nothing else.

I went one further than you, as I limited all my running for the first two years back into the sport to trails and/or the non-repetitive stresses of off-road running. So I would take my bike and park my car at a trailhead, ride for two hours on the roads, and then change out of my bibshorts back in the car, store everything out of site (minus the bike), and immediately run the trails. My "transition time" was never more than 3 minutes.

The protocol worked great for me after a very, very slow buildup of running mileage supplemented by those extra cycling "miles". I eventually transformed to multisport/duathlon, and a 2-hour ride would never again seem like a Long Ride. Even a 90-minute run would never again seem like a Long Run.

Thanks for all the good insight. I went ahead and did the workout no issues. I switched the bike to a 1.5 hour workout. And felt great on the run. It was a great opportunity to work on my nutrition as well. Also was able to stay off my feet all day which is helpful. I definitely wouldn't make a habit out of these, but I think as a confidence builder I think they are good to do every now and then.

I think if you just swim the following day you can do both together. So my coach gives me one "dress rehearsal" type brick for a 70.3 and also one for a full Ironman. For the 70.3 I might get a 3-4 hour bike followed by either a 75 to 90 minute run (depending on what time of year I am doing the 70.3). For the full Ironman I'll get one weekend with a 5 hour bike followed by a 2 hour run. That is hard especially when some of it is at race pace. But I usually only swim the following day. Just be careful and listen to your body. I probably would not attempt these workouts without my coach giving them to me, but I have only been doing triathlon for a few years.

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