As with being successful at anything in life, becoming a successful referee - and being promoted to National Panel and even WR International Panel - will take goal setting, dedication, training, practice, humour, style and loads of hard work, honesty, self analysis, and the ability to pick yourself up when it all goes 'tits-up' and you have a bad day at the office.
Nothing comes easy, especially in an 'elite' sport with a pyramid structure.
Wayne Barnes started at 15 and reached the National Panel at 21, so by all means harbour the ambition. However most referees never get that far, but still find it a very satisfying way to spend a Saturday afternoon or Sunday morning. As already said, you need commitment and talent. Make sure you enjoy the journey regardless of how it ends.
Welcome to the site.
I'd add- worry about what you can control. You can control your fitness, your law knowledge, things like that.
There will be things that you can't control - demands of work conflicting with rugby, perhaps. You need to let go of the angst of that.
And make sure your partner is supportive -refereeing is a big commitment, even if you aren't Elite-bound. Someone who will support you in your endeavours will make life a lot easier.
Lots of good advice here Joeweston - and don't worry about the written stuff. We can all understand what you write, and there is no requirement for written perfection in refereeing.
I would mention that you need a very strong backup plan in case you don't make it to the top. There are only about 50 National Panel referees (or above) in the whole of the UK, and very few of those make enough from reffing to give up the day job. Some have found work with the RFU to enable them to earn a living from rugby, but the vast majority even of the Top 50 do a day job and fit their training and reffing around it.
If you think about the numbers, there are about 50 paid professionals at any pro game - and only one of them is a referee. It is much harder to get high enough to earn a living in this role than it is in any other aspect of pro rugby, so the importance of a backup plan cannot be overstated.
and don't argue the toss with your assessors, mentors, coaches, whatever they are called these days. Take all feedback on board, discuss, digest and use what is beneficial. You don't want a reputation as a know-it-all.