When I returned to riding I made slow but steady progress relearning the basics on twice weekly lessons over a couple of years. By the end of that time I was safe to ride a well broke lease horse trails and arena w t c with enough coherence that an owner would agree to a lease :). I was a good enough mostly Western rider as a teen, went back to h/j lesson after at least 20 years out of the saddle.
I didn’t have lessons as a kid. Without lessons you need hours in the saddle to get your basic balance and work through various bad habits typical at each stage.
With lessons you can make much faster progress if you have a good coach who is able to see and correct your position appropriate to your level.
In dressage try to find a coach who is attentive to rider as well as horse. I have some friends who are allowed to do just about anything as far as seat as long as they get the horse “round” which may be strategic, dressage is not judged as equitation, but isn’t great for developing riders.
A lot will depend on how good a rider you used to be, how much remains of your skill, and how easily you adapt to the new saddle, horse, and way of going. Some folks can ride in almost any saddle, others need a saddle to compensate for physical issues.
If you have the luck to get lessons on a big moving school master you might get thrown around a bit at the start and be riding much less well than in your former discipline. And if you are in a very constructed saddle if the blocks aren’t right for you that can unbalance your seat.
As far as be taken seriously, don’t worry about that. If you are enjoying yourself and coach thinks you are making progress, don’t worry how you look to the rest of the barn. I’m not sure barn folks even notice a two lesson a week student. Dressage is much more one owner/one horse than some other disciplines where full leased or owning a string of horses are more common.
People will take you seriously for about 24 hours after you unload your new firebreathing 17 hand 4 year old $50,000 Oldenburg “prospect” and then they will go back to muttering that the horse will kill you :). In other words, don’t worry about serious as far as it means the opinions of those around you.
I also think for us adult returning riders particularly when we have success and authority in our careers, it’s important to embrace “beginners mind” like Buddhists say, come to it with no ego and no frustration that “I used to be a good rider, what happened?” Otherwise you will be self conscious, frustrated, embarrassed, and that will impede your progress.
I feel like being serious about rriding for me included being completely realistic about where I’m at, about making slow but steady progress, but also about making sure it is financially, physically, and emotionally sustainable. If one or two good lessons a week are sustainable for you then that is taking it seriously, more than having big expectations that go poof.
When I returned to lessons I didn’t think owning a horse or riding every day was a realistic goal financially or timewise. Over the years I figured out how to make that happen with resources and support I never knew existed. Continue on your journey and see what you find!