Looks very nice, and fresh concept.
- You can “simulate” progress when out of focus by marking a timestamp regularly while in-focus or online (every second), and then if the game detects a mark after a longer amount of time, fire some logic that covers more time, like update(300seconds) or update (1000ticks).
- As a mechanic, the incline of the mountain could increase, requiring more strength to push up at the same speed. That way, you might have several attributes influencing progress.
- The +X% pop-ups are amazing, wish more games had that.
Beautiful graphics too!