You’re about the same age and weight as me. I would say for adult amateur rugby, cardio conditioning matters a lot more than strength. Rugby is an endurance sport and it is much easier/more fun when you’re fit. There’s a surprising amount of people in my league that neglect cardio. So, endurance stuff like jogging, cycling, swimming etc. are great. Also interval training.

For the times you do lift, focus on legs and glutes. So many of the injuries and penalties in my league come from people’s legs getting tired and they don’t squat down far enough to tackle with good form.

For learning the rules, watching the pros play on TV is great because the ref is usually mic’d up. Anytime they say something you don’t recognize or understand, google it. You’ll learn the bulk of what you need to know in just a couple games.

Have fun! Rugby changed my life for the better in so many ways

I commend you for wanting to start so late on. Suffice to say, you need to get used to the hits and the contact especially if you intend to play competitively. I've met many people who take up rugby relatively late on, get properly hit and just sack it off and never play again. For your experience and size, I'd suggest trying as a blind side flanker (in the pack but away from the direction play when in scrums). Its a relatively simple position in that your job is to support your ball carrier and tackle the opposition when they have it. Support and tackle, thats all you do. Some of the best flankers of all time (Neil Back, Phil Waugh) were smaller guys, though the game has come a long way since them. The hits though, they can be a real shock to the system. Don't dip out of the contact sessions in training, hit tackle bags, run into people with the ball. Go in 100% because otherwise you'll end up hurting yourself! It is certainly a mindset. Just ask your training buddies for help too. Doesn't matter how fit you are. If you can't take the physicality of the game, you'll fail.

Congratulations! Nice one indeed. I've no advice, having never actually been on field. But training is tough, so attend religiously, practice ball handling often, and volunteer at your club