PEA RIDGE, Ark. (AP) — Walmart and Amazon are racing to speed up online order deliveries in rural areas of the U.S., a rich source of untapped sales that major retailers long wrote off as too sparsely inhabited, too remote or too impoverished to serve profitably. Walmart has a running start in the contest to build a loyal customer base in rural America. Roughly 90% of U.S. residents live within 10 miles of a Walmart store, and 45% of the company’s full-service Supercenters are in places with