Follow somebody into big jumps to gauge the speed or just watch where they drop from, their approx speed and how many turns they make between features.
There's a wide range you can take most good jumps. You really need to dial in your ability to hit that point or you're going to get hurt. If you're riding the same park figure out the way the jumps flow, figure out your drop point and know if you need to sit on your skis or speed check based on how you land the first. IF the conditions change check the speed again, watch somebody hit it, follow them in or whatever
Seriously, if you constantly overshooting or casing jumps that's not good for you. I'm not saying it's going to kill you but your body likes landing in the sweet spot so aim for that. If there's a jump you consistently take long, aim toward the sides of the knuckle to lengthen the distance you're traveling.
As far as your knees, hope they hold up long enough to reach the drinking age. Then acquire beer to drown out the pain.