。
After walking for about an hour, we came upon the lake.
Literal
Approximately 1 hour walk-[cond-と] we [topic-は] lake [gen-の] place [to-に] came.
約 is the formal/written prefix for 'approximately' (约 in simplified Chinese, same meaning). Note this is a different 約 from the 約束 homograph. The conditional ~と used here creates a discovery/narrative nuance: 'walking for about an hour, (as a result / as it happened) we came to the lake.' This narrative use of ~と is common in literary writing to describe unexpected or subsequent discoveries.