I know the first advice I will likely get is to search the forums, but after doing so I am even more confused. I have started commuting to work (7 miles, mostly uphill one way). I feel like I'm doing good considering I haven't been very active for a few years. I've been using strava to record my rides, and to see how I'm progressing. As I've only been riding regularly for two weeks now I certainly don't expect to see real progress yet, but I'm here looking for a bit of guidance.

Firstly, it seems that my legs tire much faster than my lungs. From what I've read here it seems like I should be searching for a good balance between depending on my lungs/a high cadence and depending on my legs/strength. So, as a beginner should I just be focusing on that?

My first goal is to be able to ride to work each day (5 days/wk) easily and somewhat quickly (right now I'm maintaining about 12mph, I'd like to maintain closer to 15-18mph).