"Thanks for the responses. I will shoot for all three sets with lighter weights."
Hi. If you have time to do your 1 Rep Max for the exercises, then you will be working with the weights that are right for you and you can make adjustments from there. When Cathe did STS she didn't mention her weights and encouraged us to do our 1RM for the exercises to get the maximum benefit from the program. However, for many of us, Cathe is the bar and we want to match what she is doing in the workouts. Realistically, some of us are not as strong as Cathe and are risking injury by trying to lift the same weights she does. Others of us are (dare I say it) are stronger than Cathe and are under-working by matching her weights.
As others have told you here, you'll get stronger as you do the workouts and it won't take long. You could start at 70% of your 1RM for a week or two and then increase to 75% and then to 80%. If you don't want to do the 1RMs, try to shoot for poundage that begins to get hard around rep 8 and really tough between reps 10 to 12. If you do 12 reps and feel like you could have done 15 or more, your weight is too light. If you hit 10 to 12 reps and have to work for them, you're at a good weight. Even if you only get to 10 reps that's okay. The next week (or when you can do 12 reps with good form) go a little heavier with the weights.
"I swear, I thought I was in decent shape, but this series is pretty challenging!"
I can relate. Before doing Cathe's workouts, I also thought I was in pretty good shape. Now, I can do workouts that I couldn't have 25 years ago. Cathe continues to raise the bar on home video fitness workouts and also on what I think I can do. These days, I embrace each new workout challenge with gusto. I'm glad to see that you and others here do the same. As Cathe says, "If it doesn't challenge you, it doesn't change you." That has been my experience as well.
RWH is definitely a challenge and is sure to give us all great results. Good luck!