I never thought much about it either until it got really bad like that. Even back when I was at UT, I noticed at one point that I developed really bad tremors - it was first really noticeable one day when I was taking a test, my hand was shaking like crazy while I was trying to write. I went to the doctor because it freaked me out (you know, you're imagining the worst, like you have Parkinson's) - he said it was nothing major, essential tremor, and that stress could exacerbate it. He also noted my bp was a bit elevated (I don't remember how high, but not crazy at the time). So he gave me a beta blocker that would control the tremor--people use them for stage fright and such--and that would also help with my bp, but that way he wouldn't have to diagnose me with it, which would look better for a 22 year old.
Anyway, after awhile, it didn't seem like I had those tremors anymore, I wasn't going for regular physicals, I let the prescription lapse. But in hindsight, that was probably a symptom of hypertension, not its own thing. So all that's to say, it had likely been steadily increasing for years, and I even had some symptoms, but they were easy to ignore. I'm fortunate that I actually had symptoms at all and that they finally became severe, because so many people don't have any, and I probably would have already had a stroke by now or something.
So to your questions:
1. With the tests, they were just trying to rule out any underlying cause and that hypertension was primary. So much of it was what you'd expect - pretty extensive blood work, EKG, some various scans. They obviously wanted to make sure nothing was wrong with my heart or arteries. The one sort of unusual thing they did was an ultrasound to make sure I didn't have stenosis in my renal arteries, which is sometimes a cause of high blood pressure in someone so young. But everything was perfectly normal. And I have an uncle who had to get on bp meds around 30, so it seems to run in the genes.
2. I'm still only 39, so your situation might be a bit different at your age. But I've been taking this same medicine for 6 years now and it's kept it under control with no known side effects. It's consistently normal (if somewhat toward the high side of normal) - 120-135/70-85. No fluctuations to the point that I don't even check it routinely anymore, and I only go for a checkup once a year now because it's totally managed. I'm actually supposed to donate blood this evening, I'll report back what it reads then for accuracy.