I would suggest doing all your rides on the road so you can get used to it.

Cycling, like any other fast paced sport, requires extensive experience to gain the recognition and subconscious reactions to ride safely in a race. Doing that when you're already in shape puts you at higher risk since you can go faster and push deeper than a typical novice. Add a race environment to that and it's a recipe for disaster. Most racing cyclists need tens of thousands of kms to build a complete skillset.

At a bare minimum I'd guess 1500 km outside on your race bike are necessary, assuming a non technical course and relatively spread out field. Yes, join a club. Listen to the guys who have been riding (better yet racing) for decades. Do all your intervals on zwift, by all means, but learn to ride fast and in control outdoors. As a bonus those skills makes you a faster racer.

I was going to say that was overkill....but then looking at 5 hours per week time budget...I'd have to agree. Shoot for outdoors unless the conditions or life absolutely prevent it.

When you say you haven't ridden outside since you were a kid, how old was the "kid" and did the kid ride a road bike, did you ride trails and do all the usual kid stuff. ( I assume you weren't on a tricycle.) There are plenty of fifty year olds' who take up the sport who haven't ridden a bike in thirty years. Buy a good lid.

Are you going to ride a Tri bike, and do you hope to stay down in the bars, which is a little trickier than a just riding a road bike. If it's going to be a Tri bike, you better go find a closed course and learn to ride on the bars with some confidence at speed. If it's a road bike, with or without Tri bars, that's a little different.

Riding in a race with a closed or partially closed course is considerably less dangerous than general road riding. If you live in central London, there's always Hyde Park, but London traffic is not a nice place to learn survival skills. And my experience of driving in England (since I left a long time a ago) is that the drivers test doesn't appear to weed out the worst drivers anymore. If you must ride, go find a riding group from a local bike shop and go out with them (on a road bike...only). So that you get used to riding with a group that's going in the same direction that you are. Spacing in a group (early part of any IM) is tricky. There used to be a few indoor and outdoor velodromes around and getting on one of those with your Tri bars is a usefull way to learn control, comfort and confidence at some speed. Google says there are a few in London. At a pinch you can ride the circular road around Regents Park for a while (which I did to shake the rust out for Worlds in 2013 )

My take on riding and racing and training is that they are two different things. Train indoors where you can control your training. Ride outdoors for a hour or two at least twice a month, to learn to ride on a real road. If you spend all your time outside you will be wasting your training time, in my, not always so humble, opinion.

Best of luck.

Do a few laps of Richmond Park if you can.... but depends which part of London you live in. It's always heaving with cyclists, so fairly safe as drivers are used to them, plus you'll develop handling skills avoiding the other cyclists.

It's about a 10km lap.

Thanks all, not what I particularly wanted to hear but still good to hear it and it makes sense!

I think I can try to do all weekend rides outdoor with a club and then go to some of the outdoor tracks in North/East London during week or whenever possible. Just needs a bit more planning.

To the question on bike.. I have a road bike that I was going to fit some tri bars too. Then if im any good and think I can keep with sport long term ill maybe look at a tri bike.

Even as a kid I just cycled round the block so no mountain biking or things like that so expect I have lots to learn.

 maybe a few trips to the velo park? theyve got an outdoor road section.

https://www.queenelizabetholympicpark.co.uk/.../lee-valley-velopark.

If you’re racing non-draft legal it’s not as critical to be an expert at riding in a group unless you do lots of group training rides. I typically ride solo but even then I practice keeping a line while grabbing a bottle, looking over my shoulder, etc. I would say at least a couple of rides a week outdoors or more if possible.

Dino,

Welcome to the sport, and good luck with your goal races. A 200W FTP at your weight and a 2:40 marathon are great numbers to start with and build from. You'll do awesome.

I don't want to necessarily be "that guy"... but my answer regarding outdoor cycling needs to be pretty clear. You absolutely need to get outdoors and ride. You need to be able to avoid traffic and obstacles. You need to be able to react and anticipate actions of other rides. You need to be comfortable on steep hills, steep descents and sharp corners.
You will learn fast, especially with an experienced group of riders.

But it would be absolutely unsafe (to yourself and to other racers) to show up at a half or a full without first learning the proper bike handling skills.

I'm new to the sport as well... but I have already seen numerous crashes caused by people who are not confident / aware on the bike.

So to answer your question # 1 specifically... a minimum of 10 long hard rides with other riders would teach you the necessary skills... the rest your rides can be on the trainer.