One benefit that happened pretty quickly was improved strength. When I rowed my first 10K, it took me over 60 minutes, but every day, the strength in my entire body increased. My bum, legs, abs, and upper body were sore the first few days, but I didn't experience any soreness after that, unless I really pushed my pace and did sprint intervals.
After week two, I felt like my entire body was stronger than it ever felt from running or from CrossFit. And maybe that's because I didn't have any low-key aches or pains. Rowing is so low-impact that I could do it for an extended period of time without my joints suffering. If I ran every day, my knees would ache, and lifting weights aggravated my old shoulder injury. It felt amazing to walk around feeling strong and pain-free, because isn't that the point of exercise? To make you feel better, not worse?
I was able to slowly increase my overall pace, which gradually decreased my overall time. On the last day of the challenge, on Day 28, I rowed 10,000 meters in under 49 minutes — my bum was definitely sore the next day! But I was so proud to improve so much in just four weeks.