A man having an affair takes his mistress out to the woods when they witness two thugs dumping a body the lake.

When a man and his mistress on a week-end tryst witness two thugs dumping a body, they must outrun and outsmart deadly pursuants through a mountainous forest.

"When they witness two thugs dumping a body, a man and his mistress must..." (do this thing)

That would be an event not a logline

For it to be a Logline, it should be something along the lines..

"When an illegitimate couple trying to pull out of the woods witness two thugs dumping a body, they learn the corpse to be his wife [and must reach this specific goal]

now (hopefully) with clear stakes, the plot thickens..