Mashing, extension and knees.

Constant mashing can be hard on knees. The other issue is leg extension. A proper setup should have you knee still slightly bent at the bottom of your stroke.
Think cadence on those hills. I ride a recumbent trike (catrike Road) - I have a ruptured c5 c6 so I can't do the head up thing on a diamond frame. I use a bike computer and almost do 100% cadence. I stay between 60 and 100 per minute. Using my lowest gear at 60 cadence puts me at 1.75 mph (it's a hill climbing machine :-) If you're mashing on the up hills down shift. If you run out of gears talk to your bike shop about getting a lower gear.

PS if you are 5'1" I suspect you might be fully extending your legs at the bottom of your stroke while seated. Lower the seat. If you do and still have to fully extend, it's time for a replacement ride. (Imo)