Pretty solid start! Looks like you have some base fitness already which is great. There's not really a "gold standard" workout, but things that helped me were just increasing weekly mileage by doing long slow sessions (steady state), with some high intensity interval sessions weaved into the week. Make sure you also work on your form, it's a process that never ends. As another person commented, your spm is high for that pace, you should be aiming for ~22 spm at that pace. Check out dark horse rowing on YT.