I have been running, I had been running, I will have had run, I would have had been running

English has a large amount of tenses as well, and if I were you I wouldn't discount the English subjunctive either

"I will have had run" and "I would have had been running" are not sensible. Perfect tenses indicate completion. If you try to "stack" two copies the auxiliary verb "have" to make some kind of double-perfect tense, it doesn't make any sense--it makes no more sense than saying "I wented to the store" or "I ated the food." There are reasons you can have utterances like "had had" in English, but it typically involves the other meaning of the word "have" (the non-auxiliary meaning), like "I had had steak, and James had had lasagna, but we both were still hungry."

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.

"By next month I will have had run 50 km" - is there a BETTER way to say this sentence? It refers to the past from the prism of next month, but also RELATES it to that point in time

"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 had run 50 km" - is there a BETTER way to say this sentence?

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.

Spanish has a similar amount of tenses, some expressed in inflections, others through auxiliary verbs. Spanish doesn't have the obsession with the gerund ("I go home" is fine), but has (im)perfect past, plusquam perfect, and future tenses, combinations (no habría pensado: wouldn't have thought) and conditionals in all, just like English. They just don't correspond 1-1.

The subjunctive in English is a bit of joke in comparison to the Spanish one.

I'm not saying English has no nuance beyond my example.

But this relatively simple grammar is English's strength. Consider how you translate your examples into other languages, especially the future hypotheticals that aren't a trivial concept to communicate period yet the rules for producing them are still simple in English.