English Equivalent of "修煉"?
KEY has two definitions for 修炼:

practice asceticism
practice self-cultivation (esp. of a Taoist)

self-cultivation has a nice ring to it.
修真 on the other hand means:

{Taoism} cultivate true virtue

修炼 has quite a wide range of meanings one of which is the same as 修真: 修行.
修真 only really has one meaning.
Although it might be interchangeable in some cases, the context is really important here.