I'm at the point where I'm running about 17.5 miles per week (3.5 miles M-F) and I want to keep increasing distance but I'm a slow runner so that 3.5 miles takes me about an hour

I'd like to get faster so I can squeeze more distance into a shorter amount of time, should I do something like replacing two days a week with a shorter but faster run and keep dialing up the distance on those days?