Basically the title? Did anyone have success with such a training method? I recently fell in love with running. I live near mountain trail surrounded by 2-2.9k meters mountain peaks. It’s surrounded by three mountains actually. Love it! I was wondering if I can substitute zone 2 rides with runs? Are there similar adaptations? It should be in my opinion as both running and cycling in that zones is increasing mitochondria. I get it, running will not make me faster on the bike. I studied physiology in university 10 years ago, but not deep enough to answer my own question. Is running increasing mitochondria in different muscles than cycling? I am a bit confused. Maybe I will need less volume with zone 2 running than cycling to see any improvements, as running is harder.
Nah I’d plan your runs and rides separately. ‘Replacing’ a 1 hour zone 2 ride with a 1 hour run just doesn’t make sense due to how much wear and tear running does to your body.
Check out barryp’s triathlon run plan (google it-- it’s on slowtwitch.com forums) and switch to a triathlon-focused bike plan on trainerroad. Your Tri bike plan will have fewer z2 rides and fewer rides overall, but plenty of intensity 
There’s two ways of looking at it:
- Run to be a better runner. Ride the bike to be a better cyclist.
- Fitness is fitness.
There’s something said for specificity… but fitness does carry over. Back in my 20’s when I was skinny, running 100 mile weeks and could run a sub 30 min 10k, I would crush the hills drawing all kinds of praise on the local MTB ride. That is until they all passed me on the downhill and saw how soft I was. 
As a coach I often get swimmers and soccer players out to run cross country. They absolutely crushed it!
But at the pointy end… it is those who are wholly committed to that singular goal.
That said very few are at the pointy end. Life is short and am all for doing what you love. If you find enjoyment in running your local trails do it!
Different muscles.
Generally it is accepted that cycling improves running, but running doesn’t improve cycling.
There’s a running thread and many triathlon threads you might get good direction from.
Keep your running short and easy, and progress volume slowly.
Maybe if you are not good at running. But elite runners I know can transfer over to the bike quite well.
That’s not quite the same thing. The general fitness from other sports will carry to a degree.
Improving cycling will come from training cycling, hardly at all from training running. What will come from running will be increased fatigue and injury risk.
Not my words, I’m just the messenger here.
I did running only for a long time and was good at it (could keep a fast pace - felt great).
I did cycling and running half/half for a year (bc of injury and to see if I would like this variety) and sucked at both in pace for running and power for cycling (however, had amazing endurance).
I’m cycling only since the end of last year and getting good at it (FTP going up).
Moral of my story: If you like both and enjoyment, fitness is your aim: do both. If you want to be good at one, just do that one.
I mix both (2x-3x bike and 1x run a week). However, my primary objective is to stay active and maintain a healthy lifestyle. No racing whatsoever (only with my past-self). 
This needs to be repeated multiple times, it is soooo true.