"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
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.
Subjunctive may be interesting to analyze, but it's not an additional form of the word.
By my count, English has 12 tenses. You cannot just invent new ones, because people won't be able to understand you. They are fairly regular: present, past, and future, which can be modified by making them perfect, continuous, or both.