The idea （主句主语）struck（主句谓语） me （主句宾语） a long time ago（主句状语） ,but（连词） not until（引导一个状语从句 ,用了倒装） now did I start to do it.
not until 引导的是状语从句,
“直到...才”的意思.下面是具体用法,涉及到你问的did的倒装.
倒装句之部分倒装
部分倒装是指将谓语的一部分如助动词或情态倒装至主语之前.如果句中的谓语没有助动词或情态动词,则需添加助动词do, does或did,并将其置于主语之前.
句首为否定或半否定的词语,如no, not, never, seldom, little, hardly, at no time, in no way, not until… 等.
Never have I seen such a performance.
Nowhere will you find the answer to this question.
Not until the child fell asleep did the mother leave the room.
当Not until引出主从复合句,主句倒装,从句不倒装.【【其实题中的but连接的第二个句子就是这种情况,not until now是从句,did I start to do it.正常的语序应该是：I don’t start to do it until now 】】
典型例题
1) Why can't I smoke here?
At no time___ in the meeting-room
A. is smoking permitted B. smoking is permitted
C. smoking is it permitted D. does smoking permit
答案A. 这是一个倒装问题.当否定词语置于句首以表示强调时,其句中的主谓须用倒装结构. 这些否定词包括no, little, hardly, seldom, never, not only, not until等.本题的正常语序是 Smoking is permitted in the meeting-room at no time.
2） Not until the early years of the 19th century ___ what heat is.
A. man did know B. man know C. didn't man know D. did man know
答案D. 看到Not until…的句型,我们知道为一倒装句,答案在C,D 中选一个.
改写为正常语序为,Man did not know what heat is until the early years of the 19th. 现在将not提前,后面就不能再用否定了,否则意思就变了.
以否定词开头作部分倒装
如 Not only…but also, Hardly/Scarcely…when, No sooner… than
Not only did he refuse the gift, he also severely criticized the sender.
Hardly had she gone out when a student came to visit her.
No sooner had she gone out than a student came to visit her.
典型例题
No sooner___ than it began to rain heavily.
A. the game began B. has the game begun
C. did the game begin D. had the game begun
答案D. 以具有否定意义的副词放在句首时,一般采用倒装句(谓语前置).这类表示否定意义的词有never, seldom, scarcely, little, few, not, hardly, 以及not only…but (also), no sooner…than, hardly… when scarcely… when 等等.
注意：只有当Not only… but also连接两个分句时,才在第一个分句用倒装结构.如果置于句首的Not only… but also仅连接两个并列词语,不可用倒装结构.
Not only you but also I am fond of music.