I've been incorporating rowing into my marathon training for a few years with some degree of success. I initially used the FIRST model (3 high quality run days, 2 cross training sessions) and ran a couple of marathons in 3:15 and 3:22 (my PR is 2:36, but that was way back in 1981) respectively; total run mileage was about 30 miles per week, total rowing was 15-20k. I've since modified my program to include more high quality rowing into the overall plan. In addition to 30 miles per week of running, I generally do around 50-60K or rowing a week spread 3 days; one session will average 20K of steady state at a stroke rate of 18-22 (usually averages about 2:17/500m), a threshold session of about 15K, to include warmup/cooldown of 3k each, 4-5 x 2K @ 2:04-6/500m, and another 15K session of warmup/cooldown of 3K each, 6-8 x 500M @ 1:55-2:00/500M. Overall, that averages out to about 100-120K per week of pretty good quality work. As long as I don't fall apart, I'm optimistic that my next marathon will be a success. At this point, 3:15-3:20 is the target. I also do 2 30 minute kettlebell workouts per week for strength. I've found that after many years and miles, rowing helps maintain fitness with less impact on my body. After the marathon, hope to focus exclusively on rowing with a goal of rowing a sub 7 minute 2K. Good luck in your recovery.