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Why do we use a continuous verb after "adjective + of",
e.g. "I am capable of running 10k"
or "I am scared of sleeping in the dark"?
When we use "to" instead we do not, e.g. "I am scared to sleep in the dark"-- why is there a change from continuous to present for the verb?
"of" is a preposition, and generally the complement of a preposition can be either a noun phrase, a content clause or a participle-gerund.
In these cases it is idiomatic to use a participle-gerund. The gerund form is "running". It would not be correct to have an unmarked finite clause, or a finite predicate, as the object of a preposition. It would be incorrect to say *"I am capable of run 10k" It is also generally incorrect to say *"I am capable of run to 10k"
