Depending on lung function / how the person feels - it can be totally realistic!
My husband is 29, PFTs around 70%. For our honeymoon this summer, we did a 10 mile backpacking trip (portable neb and all) on Isle Royale. For our annual trip in September, we did an overnight trip to a chalet for a total of 12 miles at 6-7,000 feet elevation in Glacier National Park. The next hike we did was a day trip of ~12 miles with a heavy elevation gain (~2000 feet in gain, topping out at around 7000 feet ).
Most important thing - he needs to drink a lot! Jeremy got dehydrated on our chalet hike, and he felt miserable. Next trip, I filled his 2L camelback with Gatorade, and he made an effort to drink more of it (he emptied it by the end of the hike). He was still sparkly with salt by the end of the hike, but he felt great.
Ground pounding (hiking) is great lung clearance, especially with a 40 lb backpack (he didn't need to do his evening CPT when we were backpacking on our honeymoon).