The older I get, the more I prefer uphill to downhill. It's the old knees. You may need to teach her how to go uphill. Slow steady breathing in time with each step will help. "Gear down" when you hit a hill. The goal is a steady heartbeat- not a steady pace. On the flat, maybe 2 steps;1breath, moderate hills 1 step;1 breath and on really steep hills the "rest step" a technique of taking one step, then a pause, then repeat. Keep a steady pace- do not stop and go. Take a 10 minute break once an hour.
I will have her try this the next time we get out. Normally she is a fast walker but with the asthma it slows her down.