Based on what you posted above, there are a few misconceptions that I just want to make sure you are aware of.

1. building muscle and losing weight are not mutually exclusive. They are two different things. What you want is to lose unhealthy weight, which is fat. I'm making a jump here, but a lot of people don't understand the difference between building muscle and bulking up like an athlete. In reality it takes a lot of very hard work and heavy weights to bulk up like that. So don't think that there is a certain order to things because building some muscle can help you right now.

2. You actually are building muscle right now with all of your walking. At 2-4 miles a day, it doesn't matter if you are 200, 300, or 400 pounds. That is weight that your leg muscles are moving around. And like any muscle building, your body will get used to the workout and will find a steady state.

3. There are so many good exercises out there that can help. So with that said, I don't recommend running for where you are weight wise. It is not good for your knees or feet or back. You would get safer cardio from doing shadow boxing/kicking, burpees, or similar low impact but high intensity movements. And the different movements will help your weight loss by forcing the body to adapt to different things.

4. You would be amazed at what you can do with just a pair of 25# dumbbells at home. You really don't need all the fancy equipment at the gyms to get a good workout.

I'm pushing just a bit of weight lifting to add for you because it will help you reach your weight goals a lot faster. And yes muscle does weigh more than fat, but you would have to build a lot of muscle for that to really matter on the scale. But no matter what you do, keep it up and congrats on all of the progress so far.