。
I can't run as fast as him.
Literal
I [topic-は] him [to-the-extent-ほど] fast run [nominalizer-こと] [subject-が] can-do-not.
~ほど in negative context means 'not to the extent of X / not as X as.' 彼ほど速く走ることができない = 'cannot run to the extent (as fast as) him.' ~ことができない is the explicit potential-negative construction. A comparative negation.