The 2000-Meter Test
This was one of the top three most awful experiences I've had with training. I'd argue that it's right up there with super-high-rep sets of squats and deadlifts.
I came in at 7:40 for my first 2000-meter test. The world indoor record right now for men is 5:36. (For women, 6:25.) See where you're at, then next time try to knock off a few seconds.
The Countdown Challenge
This is a low-volume, shorter duration protocol that's a bit easier to suffer through as each successive set is slightly shorter than the next.
Row 500 meters
Rest 1 minute
Row 400 meters
Rest 1 minute
Row 300 meters
Rest 1 minute
Row 200 meters
Rest 1 minute
Row 100 meters
Your Favorite Song For Distance
If you have a few favorite songs that always get you fired up, turn one of them on and see how far you can go during that one song.
Music is a great motivator, but it can also be a great timer for bouts of intense exercise, as not many songs last longer than five minutes. Much like you might have personal bests on the bench press, squat, deadlift, and other exercises, you might have personal bests for distances on a few different songs.
30/30 for 10 Minutes
It's simple: 30 seconds of hard rowing, 30 seconds of rest, repeat for 10 minutes. This is another good option because it goes by quickly, and none of the work bouts are so long that you'll get bored out of your mind if you're someone who hates cardio. The only downside is that the rest period is so short that you barely have time to stand up before you have to get back down and start rowing again.