Ouch, a 1/7 combination is completely cross-chained and not advisable for long periods of time. You should be more like in a 2/3 or 2/4 combination for usual riding. My MTB is 3/8 and my most often-used combination is 2/4. My chain stays there probably 60% of the time or more.

As for speed, I'm still slow and usually average between 12 and 13 MPH moving average according to Strava and the bike computer, but I have ridden as much as 44 miles in one ride.

I think you really need to learn how the gears work, and when to change them for climbing vs. descending hills. Move the right shifter (rear derailer) to a lower number for a lower gear to make it easier to pedal uphill, or to a higher number or higher gear to pedal faster downhill. If you get too close to the extreme upper or lower end of the range on the right shifter, then move the left shifter into the next higher or lower gear range. You really want to stay off the 1/7 combination, and also the 3/1 combination. That makes the chain have to bend pretty far and puts inordinate amounts of wear and stress on the drivetrain.