I disagree with @zzazaaaz, I like the pacing, it gives a rush to mine, different from the others where it's one long session. (But not too different, because it basically starts with short sessions in basically every incremental, here it's just a bit more, but I like it)
Besides, it's just the first level, I'm sure you can last longer the deeper you go or you get another checkpoint or something.
Edit: Also, the middle branch is very useful, the 10% damage upgrade there allows you to one shot copper.
Just one thing, I do think it's nice if there was an "early return" button or something like that, and auto battery charge when you reach the surface.