Hi tbtbstressed,
Wow! Thank you for the kind words! I am very excited to hear about your amazing accomplishments...you should be so proud!
So, to answer your question in regards to including strength training into your week, YES, you should! Performing high intensity cardio workouts such as step aerobics and spinning is a great way to burn a lot of calories and create nice, lean looking muscles. You are right that these workouts do tend to focus more on the front of the legs (quads), and include a lot of forward movements, which may leave your hamstrings somewhat weaker. Typically, women are not going to build bulky muscles b/c we simply do not have the hormones or genetic make-up to do so. But muscular, shapely legs will be the positive result when you are eating right and exercising!
Strength training is critical to the long-term success of any weight loss program and will promote a more efficient metabolism. This is b/c muscle is more active than fat, requiring more energy (calories) than fat, even at rest. I would recommend that for best over-all results, you not only focus on the legs, but perform an full-body workout! Work your quads, hamstrings, gluts, back, arms, etc. etc. I would also encourage you to begin adding in a day of yoga or pilates. These workouts will keep you long, lean and flexible.
All you need is 5 days max of 1 hour per day of cardio. Anything more than that can actually thwart your efforts. The fact that you're including bursts of higher intensity is a very good thing, so keep that up!
Best of luck to you and keep us posted with your progress,
Kelli