Your back should straighten as you gain core strength and hip mobility. You might help your body keep a proper posture via something like holding onto a sturdy object in front of you. The best way to make improvements is to be thinking about and moving toward proper form. For example, focusing on where your weight is, trying to engage your abs, thinking about your shoulder blades pinching together and having a long back. You might try widening the stance and leaving your toes a little less externally rotated, or just play around with shifting your weight, using an object to assist you into a proper position and then slowly working toward using less and less assistance by transferring more weight into your legs.

30 minutes seems crazy to me! How do you feel coming out of it?