Before I used to meaning?

Solution 1:

"I used to run marathons" means that during a period (or periods) earlier than now I ran marathons.

"Before I used to run marathons" refers to a period before the start of the period of marathon running.

"Before I used to run marathons, I was lazy and inactive".

Similarly:

"I used to be a lawyer" means that during a period (or periods) earlier than now I was a lawyer.

"Before I used to be a lawyer, I was a builder" may be part of a narrative such as "Before I used to be a lawyer, I was a builder. Now I am retired." In this case too, the lawyer period follows the builder period and the lawyer period ended before now.

Finally:

If the lawyer period continued until now, it would be more usual to omit the used to and to say something like "Before I became a lawyer, I was a builder"