Singlespeed touring/bikepacker here in the PNW, so we have our share of hills. Although the goal of touring is not necessarily speed, the 100 miles per day with 6000+ feet of climbing is not uncommon.
Your 48/17 is an impressively large gear for the distance and terrain you are talking about (at least for me). For road touring, I ride a 42/16 (fully loaded front/rear panniers). For offroad bikepacking, I ride a 32/18. I have found these ratios work best for me.
Experience/practice in different terrain and gearing will probably lead you to your answer. My go-to training ride in the area is 25 miles with 2500ft of climbing. After each ride, I try to think of how i felt on each key part of the route, what i liked, what i didn't, and where i could improve. Sometimes i find myself racing groups of road-bikers uphills for fun just to get passed by them on the downhills
As for gaining distance, again, knowing how to pace yourself on uphills and false-flats/headwind is definitely key.
What is your overall goal? Distance or speed?