Your goal seems just a little out of reach at this moment, but don't focus on that. Remember you're only 17. You're still growing and you'll get faster (as long as you don't break yourself). You've got the right attitude asking for external feedback and staying focused on your short/mid term goals. You will get there, probably sooner rather than later.

1) Eat boy! Try put on 2-3 lbs a month. You're a little light for 6'5". At 17 you're just getting into the age where you can really start to gain strength. Do you have a strength trainer? If not, get a trainer. Make sure they understand rowing and young people. You don't want to be going too heavy at your age (growth plates!) or doing something stupid. Volume with emphasis on good technique to build mass, not 1RMs.

2) As people have suggested, cross training like biking or running as opposed to just steady state will be great for your cardio base right now. Of your cardio sessions (6-10/week, yes?), maybe aim for 1-2 of them to be something other than SS. This early in your rowing career cross training can still help you with your rowing goals, prevent injuries, and stave off burn out. Look up the Rowing Canada "Long Term Athlete Development Model". But ya, otherwise moar SS.

3) Sleep and take rest days. Have non-rowing friends. Super important.

4) Try attacking the start a little harder (e.g. first 20 strokes) and aim to be a little bit below target for first 500. Then go a little easier through the middle (it should average out though). Right off the start you use an energy system that's different than the one you use during the middle of the piece. So you can expend this system on the start, gain a second or two, and leave a little bit more room for your aerobic system. Theoretically this anaerobic system will "recharge" a bit during the row and should be available to power a sprint at the end, as long as you're not completely gassed.