The lift was out of order

[Bassim]

I am wondering if my sentences sound natural.

The lift was out of order, and Bob had to climb ten flights of stairs. When he finally reached his flat, he was breathless, as if he had climbed a mountain.

 

[emsr2d2]

A little like one of your previous examples, I would change the comma and the word "and" to just the word "so". Presumably, Bob had to climb the stairs because the lift was out of order. That gives you cause and effect making "so" appropriate there.
At the end, I would probably say "feeling as if he had climbed a mountain" simply because presumably he doesn't actually know how he would feel if he had actually climbed a mountain.