Today, the weather is much cooler. It can be said that it is becoming autumn then winter now. I haven't run for donkey's years and now I really want to try again. I never heard of "donkey's years" before. I do not understand your second sentence. Could you explain it to me?

Today, the weather is much cooler. It can be said that it is becoming autumn then winter now. I haven't run for donkey's years and now I really want to try again. Well done for DONKEY’S EARS! A fantastic expression! No corrections needed although ‘it can be said’ is very formal. We’d probably say something like ‘it’s really starting to feel like autumn’

Today, the weather is much cooler. It can be said that it is becoming autumn then winter now. I haven't run for donkey's years and now I really want to try again.