My two have been completely different in terms of walking.
Joshua walked at around 14 months and that was it, he wasn't bothered by the pushchair. He wanted to walk everywhere. By 18months old, he was quite able to walk around the woods (around 2 miles route) and not be phased by it.
Jacob, walked about the same age but a total lazy little git. He would still use the pushchair now if given the choice.
We have spent the past year building up his stamina. As a family, we go on lots of walks/hikes so we have started on gentle short walks and built it up. He managed a 5 mile hike a couple of weeks ago (very steep in parts) & he did brilliantly. Started complaining about 0.5miles away from our house that his legs ached (understandable!) but didn't asked to be carried.