IOWA CITY, Iowa — Terry Wahls can walk. She can ride her bike. She can run two miles on a treadmill. She can tend to her backyard garden, which on an unseasonably warm April afternoon in Iowa City, is in the chaotic throes of the early Spring growing season. Wahls gives a tour, her 14-year-old dog, Teddy, trailing: Persimmon and pawpaw trees, thornless blackberries and raspberries her grandchild will pick when they ripen. The peas have sprouted…