wrong wrong wrong wrote:
Completely wrong.
If you can only run 15 minutes for your second run, you will benefit from it. You will not benefit as much as a 30 minute run but you will benefit.
This is correct. Even a 1 mile jog each morning will make you a better runner, and at any total mileage level.
I don't know the exact mechanisms. Perhaps it just helps you absorb your other workouts better, and so you'd not count it as a beneficial training effect. No matter what you want to call it though, it helps people get fitter and race faster.
For runners just beginning with doubles, it can be most beneficial to keep the double at less than 25 minutes. A rule of thumb that I set out for these folks is to make sure that they're still doing whatever would be best for training in their primary run with no regard to the doubles, and get in a secondary run that doesn't take anything away from the primary.
The duration and intensity of that secondary run may increase as one gets used to them, but there's no reason to rush that process. Just start getting in the doubles.