New Where to start?

I think you've got a great start!

Generally speaking swimming is the biggest limiter, but also the shortest part of the triathlon, so shouldn't take too long to get comfortable enough to survive that.

After the 6 weeks, I'd look into a master's swimming program, ongoing workouts that scale to ability/speed, with coaches generally on deck to help out. Helps to really get long term consistency and improvement.

Do you already have a bike? The aerobic fitness itself from marathon training should help carry over to the bike, but the muscles used are different, and handling the bike will probably be the toughest part to get used to.

If you just want to finish about any bike will do, but getting a road bike TENDS to be the best bet starting out, it's pretty versatile, can do group rides with it, and it's fairly efficient.

Practice your transitions, and running off the bike.

Wou shouldn't have any trouble finishing your first and then starting to see where you can improve to really race them!!