The weight you use is up to you. In your case, I'm sure you'd rather build muscle endurance as opposed to strength. Keep the same amount of reps and sets, or you can drop down and do three sets of 12 on bench press and squat. As far as your leg extensions and leg curls go, increase the repetitions to 20 or so. Lighten the weight if you need to. The entire idea for you is to build muscle endurance, as opposed to brute strength. It can be obtained by increasing repetitions, and doing less weight, as opposed to doing heavy weight and less repetitions.
