i can only echo what everyone has said. i was in a similar position as you... i ran 4:27 but not good enough to make the top 20 d1 program at the university i attended. i also hit a major growth spurt as a senior in high school and didnt run well/ outgrew competitive running with broad shoulders, etc.
my natural rowing weight was around 168-- i got down to lightweight when it was necessary for the team, or ate whatever and lifted whenever when i was in a heavyweight boat. it was a great way to continue in a challenging and team-emphasized sport. i still have tons of memories of just being out on the water, surrounded by trees, at dawn or sunset.
i found the workouts to be very intense. running gave me a leg up on the other freshman at first, who weren't used to pushing themselves of pacing themselves. but 14-16x500, 8x1000, 4x2000, 2x5000 were all incredibly taxing aerobically and anaerobically. i agree with the previous poster in that i have never hurt as much as i did in crew races, erg tests, or workouts. i remember my third head race ever, a 5k, i couldnt feel my arms the last 1000 meters or so. i wasnt used to being anaerobic for so long. because there is less stress on the muscles, coaches may do more interval workouts per week than in running. we were either doing intervals 2x a week, steady state 2x a week, a race and 1 or 2 recovery rows. and the weights after a workout... that is a new type of pain.
in terms of advice now... if you know you want to row, get on an erg from time to time and practice getting up to 40minutes at a good pace and 60minutes at a controlled pace. you could throw in some intervals of 2x10 minutes, 5x500 to get used to rowing at a higher tempo and faster pace. do your steady state at around 26-7spm, your controlled at 22-24 and your intervals at 28-30. work on your form on the erg. watch some videos. the ways guys in the gym are doing it is not the proper way to do it, most likely. if you can, get out on the water for some intro lessons. a lot of rowing clubs do learn to row classes a few times a year for a few hundred bucks. this would be extremely beneficial. basically, if you want to make the top novice/ freshman boat, the decision will be made based on your form and your erg time. making it in the first boat will get you more attention from coaches and put you on a track to make varsity down the road. in my experience, the people who quit tended to be the people who were less invested, not on the first boat, etc.
i went to a club team where funding was always an issue. this made crew more time consuming than usual, as we were always fundraising, traveling in shitty vans, etc. i would look to teams that either have varsity or a well supported club team. UNC, UVA, Michigan are all well-funded club teams that compete at very high levels. A UNC team we lost to in regular season meets ended up placing 4th at IRAs my sophomore year.
i know this is long, but i hope helpful. i would certainly recommend rowing, and can second it keeping you in great shape. we often went for 7 mile runs during the summer, offseason if we didnt feel like erging and were running under 7 minute pace and would hammer at the end. i got down to a 6:24 erg my junior year before breaking my hand my senior year. so you can definitely be very competitive as a lightweight. in fact, a taller runner i ran against in high school (sam burns) ended up rowing in Washington's varsity boat, so it is possible to be top in the country with hard work. good luck.