Certainly foot soldier valleys are a safe target. Never send archers alone. Use 1 pike and 1 sword for safety. Losing archers is never fun. Also 20K archers is a bit on the lean side. Nearer to 30K is better. Hitting a valley with sword and pike, plenty of, likely will lose archers.
Over 5 miles, the round trip with ballista, is over 30 mins, which is over the refill time.
Scouting 16 by 49 squares, will normally yield 5 good valleys, typically 4 foot soldier and 1 archer, that is enough if persistent to collect medals over time.
Listing the valleys which are owned is good thing as well, saves time.


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