Awesome! Well done.
– For my normal runs (90? – 120?) should i run them not so close to my Z1 top?
Yes, absolutely. I’m the same. Most of my training is AeT -20%. I call this “Zone 0”.
Here’s why: As the aerobic system improves, heart rates stop changing, but speeds continue to increase. Zone 2 starts easy, then becomes hard. Zone 1 starts easy, then becomes hard(er).
As the speeds increase, the “easy” training is no longer easy. Lactate is still low at those paces, so it’s metabolically “easy”. But the speed creates more and more demand muscularly. Eventually (and it sounds like you may be there already), AeT speed is almost as tiring as AnT.
So by all means, train in “Zone 0” if you think it will help.
– Downhills: should i run them easier in general? I tend to run them “fast” almost in all my weekly training runs.
I think it depends on the stress. If you’re trying to build volume, slow down. If you’re trying to sharpen for a race, speed up.
– He’s not doing any speed work but getting faster and faster. Is this due so much volume which recruits mostly slow twitch muscle fibers, till the fast twitch has to take over?
As I said above, the “easy” training becomes faster. I’m not sure why. My guess is that there could be some fiber recruitment and/or the slow-twitch fiber, unaided by the fast-twitch, has to get stronger? (I’ll ask Scott J. to comment.)