The first time I hiked a mountain I needed help along the way, I had to take long breaks and finally near the end, I was exhausted and had a fever. (Thankfully I made it)
I had never put my body under that much stress for a long period of time before ever. But when I woke up the next day... I felt stronger.
The next time I hiked a mountain I was a little better at it, still horribly slow, but a little better. I wasn't exhausted at the end and was actually able to drive 100+ miles afterwards.
I hiked a mountain last week, the longest hike I've ever done (Distance and Time) and yet, I felt incredibly stronger than any other time.
You get stronger over time.
This doesn't just apply to hiking mountains.