I don't think so.
After a certain point, your fitness will start to plateau if all you are doing is JRA.


For example, if I was to ride less than or equal to 3-3.5 hrs at a z2 pace, I would feel like I am not stressing any of my energy systems and just burning calories or "keeping the legs moving" if you will.

I starting riding about 3 years ago through the fixed gear scene in San Diego and at my school in norcal and was primarily a runner; I have been training and competing on the bike for almost 2 of those years now. I already feel that if I am riding less than 3 hours at a steady z2 pace that I'm not really stressing my body.

That doesn't mean I don't ride like that for fun or other reasons from time to time, but my body is already used to bigger miles atleast twice weekly.

When I am riding for "training" purposes these days I don't really plan any rides between 2 and 4 hours and there is a significant intensity difference between most 2 hr and 4+ hr rides for me.