True races are always 2,500 meters. These are head to head races, 5 or so boats lined up next to each other.,starting from dead stop. Horn blows and you go balls to the wall until you have nothing at the finish. The long distance races 5k, 10k, etc aren't really races. At best they're a time trial. Rolling start, one at a time, different conditions for each boat etc. More for sport and something to do in the fall to stay in shape.
Generally your stoke rate doesn't change much if you're balancing the anaerobic vs aerobic line correctly. Most likely it will be your split that changes based on distance, stoke rate less so. But I will adjust things mid-session depending on what type of recovery I'm trying to accomplish. If your legs get fatigued you might lighten the power for 10 stokes and increase the stoke rate to maintain your split. If your lungs are getting gased, you might increase power for 10 stokes and lower the rate to recover. Or lower both. You would never do this on the water in a boat because there's other guys rowing with you in synch, but rowing solo on an erg allows you to react to the conditions of your body more. Similar to cycling, you're encourage to stand on occassion vs always sit to change the emphasis on muscles in your legs and allow for slight variances in recovery.
I'm not a fan of long distance rowing unless you're trying to loose weight. The intensity is often too low to really make you stronger and you get most of the value of the workout in the last minute when you decide to push it. Better off doing 2-5k, but row hard and push the limits of your body. You'll get stronger and gain more aerobic capacity faster.
Form..power stoke:.give it a good reach forward (knees almost in your chest), arms level, lock your back up firm. Explode with the legs first, then transition to pulling with the back and finish by pulling with the arms. Finish with the hands coming to the center of your chest, stopping an inch short of the chest. Too many people usually try to do it all at once and not as a sequence ...and/or they simply shorten the their stroke. From a stats/numbers perspective, rowing with a long stroke is better. You'll see a difference in your times for sure.
Recovery portion: everything in reverse order. Try to have fast hands or getting your arms out from your chest and straight again. Then a medium speed on moving your torsos forward and lastly a slow slide forward. Your legs sliding forward will seem so slow compared to how fast you went back. Don't shoot your butt forward, get your arms and torso over them first, then slowly bring your butt forward back up into the start position.
Have fun. Track your stats and try to beat them. Even better, ask the guy next to you if he wants to race side by side. For 1,500 meters. That's when you'll really see a difference because you can see his form, splits, stroke rate etc. it's pretty cool when you appear to be going slower and yet the moniter dash is saying you're going much faster than him. ....and he's like WTF, this thing is broken!