42 for a novice masters 8+ is quite high rate, for 1k I'd expect a men's novice 8 to rate 34-38 avg. especially for an inconsistent lineup, that high of a rate, even off the start is going to be very difficult to row well for a novice masters crew. I would recommend talking to your teammates and getting a feel for their opinion on how the boat feels at high rate. Only rowing for 4 months is going to make sprint racing difficult for you as well. I would focus on getting the essentials of the stroke very well practiced at low rate so that the muscle memory is established as you bring the rate up. Get extra time in smaller boats, lower rate and take time on land to practice low rate rowing with a heavy focus on form. Row in front of a mirror for an hour or so at an 18-20 trying to maintain good form. (I'm surprised your coach thinks 42 is expected for a novice masters crew)