I'm worried about muscle loss as well. I've read that muscle loss is normal during weight loss, but to maintain or have a less severe decrease you need to lift heavy things. I was informed of a workout regiment from paleobird regarding a workout from Mark Sisson called Primal Fitness. It's free on this website, so check it out. I read through it and it's actually a really good program. You lift heavy once a week by using body weight type workouts, push ups, squats, etc, with several levels from easy to hard. The remainder involves moving around by walking, riding a bike and doing something that's fun for you. He also recommends sprinting once a day, but that is also a routine with levels.
For me, I lift heavy for 15-20, do a short 30 minute yoga routine, and hike once a week. I would like to incorporate sprinting into my routine as well, but that will happen when I feel comfortable. I have run up a couple of the sections when I'm hiking, so that may count.
You may want to consider the following for yourself... My primary focus is on eating properly, losing fat, and becoming healthy. Once I reach my goals, I'll focus more of my energy to gain weight through building muscle. Be patient, it will all come together. You'll find what's best for you.