Y'know, we never did anything special to train the dogs for hiking. Just like humans, be wary of big challenges without preparation; if you get off the couch and try to run a marathon, you're gonna get hurt.
First 2 times, I concluded a corgi was good for only 9 miles or so, then they'd just sit down and say, "Carry me". Then I found the sore that the chest harness was wearing on the collarbone.
Ours are mostly couch potatoes, but I try to give them two city walks a day, over a mile, and FAST, once the sniffing is done. One thing I do think is valuable: we poach on the paved school playground (No Dogs Allowed At Any Time) when we can get away with it, on best behavior, and they LOVE to chase a soccer ball around (current favorite is a child's basketball, lighter, smaller than a soccerball, and toothproof). Al particularly turns into a Total ANIMAL with these balls. They'll do it forever. This gives them some aerobic exercise. They come home panting, for a long time. That's good.

Gwynnie's not happy with me after, say, the 19-mile mark, but she's gone as far as 23 in 14 hrs with 9000' gain and some off-trail. May take some prodding, but if they have to keep going, they understand.