你跑步很快 is a [SVO] sentence
你 (you) 跑步(run) 很快 (very fast)
很快 (very fast) is an adverb describing the verb 跑步 (to run)
你跑步跑得很快 is a [topic + comment/opinion] sentence
Topic: 你跑步(you run)
Comment/ Opinion: 跑得很快 (run to the degree of very fast)
Within the comment sentence: '跑'(run) is a verb; 得 (to the point of) is a degree complement; 很快 (very fast) is the adverb
Other example of how 'degree complement' works
Topic: 你說話 (you talk)
Comment/Opinion: 說得很過分 (talk to the degree of excessively)
Note: the comments: 跑得很快 and 說得很過分 are SOV sentences with the degree complement 得 before the adverb to indicate this adverb describes the degree of the verb.
你跑步很快 (SVO) and 你跑步跑得很快 (topic + comment) means the same thing, -- "you run very fast" ; "you run to the degree of very fast".
The [SVO] sentence put emphasis on the verb '跑步'
The [topic + comment] sentence put emphasis on the topic, '你跑步'