Our Border Terrier is sadly retired from really big walks - he's 13. He's done 4-5 miles recently but is, forgive the pun, dog tired afterwards. He started to struggle on rockier walks, whether that was his pads hurting or arthritis in his joints we don't know. He got carried a couple of times. He always needed a bit of assistance with big steps be they up or down. He struggles to get on the sofa now unless you're eating a biscuit when he's like a rat up a drainpipe.
I realise none of that helps the OP - would just echo what others have said. I guess it's the same as humans - don't take a dog or a human on a 20 mile walk if they've never walked more than 2 or 3 miles. The dogs won't moan as much though. Breed is clearly important - Border Terriers were bred to keep up with a hunt, so much larger hounds and horses. He'd go on 15 mile walks with me and probably do about 25 as he was constantly racing ahead then doubling back to see where I'd got to. I used to take a collapsible water bowl for him, he was happy to share my sandwiches so a few treats stuck in a pocket was all I needed.