2K is 80% aerobic. Only 20% anaerobic. To hit 7 mins, 300 watts, you need roughly 240 watts from your aerobic system. A 40 minute 10K is a 2:00 split, is nearly all aerobic, and is 205 watts. So you are looking for another 30-40 watts aerobic base. 240 watts is a 1:53 split.
How are you training your aerobic system?
Couple of approaches to use for aerobic. I use the approach Elizabeth uses -- lots of long/slow (45 to 100 minute sessions) with at least one and up to three hard session a week. There are other training approaches that work.
Expect gradual gains, maybe 30-50 watts/year at the start, lower gains after the first year until you hit your limit. If you look at training plans for improving cycling FTP and find one you like the same training approach applies to rowing and will give your your aerobic base.
Note, you can train your aerobic system with exercise other than rowing. Running, cycling, elliptical, hard hiking, etc will all build your cardio system. That can take some load off your back and keeps interest up. A bluetooth headset for music or movies or audio books also helps on long rows.
PERSONAL preference. Stop targeting 1:45. Do a 2K at 1:55, should be a solid but not extreme effort. Then slowly crank down the time on future efforts. Train to success, not handle down, by using objectives you can hit with 95%-98% effort, not 100%. Handle down is brutal.