By next month, I will have run 50km. This means that "I run 50km" completes first, and then next month arrives afterwards.
> By my English textbook there's 16 tenses, 4x4
Depends on how you count tenses. Different textbooks will give different counts, depending on what is considered a "tense". Which is not a reflection of the complexity of English, but a reflection of how hard it is to come up with definitions for terms in linguistics. "Would" is often not counted as a tense, but it may be counted as a tense, particularly if you are explaining English to speakers of a language which have a tense expressing something similar.
> Point being, English grammar is far from simple, as this conversation clearly demonstrated
Most languages are far from simple, I don't think English deserves special attention.
"If I had continued boxing professionally, I would have been running 50 km a week" Here's it's a conditional, I think I stuck an extra had in there by accident.
By my English textbook there's 16 tenses, 4x4
Past, present, future, future-in-the-past (would) x Simple, continuous, perfect, continuous-perfect (have been V-ing)
Point being, English grammar is far from simple, as this conversation clearly demonstrated