Why do some older women have pain and get tired after walking alot? Are they just sore and don't know it?

I know one 65yr old woman who had lots of pain in her hips and legs but her muscles are so atrophied from a lifetime without exercise. What is causing her pain and what can i do for her? Another woman in her sixties is walking 45mins a day since her husband died of heart complications. She want to avoid the same fate but she gets so tired after her walks. What is going on? Shouldn't walking make people feel invigorated and happy?