I'm following a general half marathon plan and running around 60-70 miles a week in singles. I'm also doubling 4-5x a week but only doing a short, very easy run of ~30 minutes (ends up being 3-4 miles).
I'm slowly increasing my singles mileage from 60 to higher up. At the same time, should I be increasing my second run too? Should I be going from 30 min to more like 35 min and then 40 min or am I getting enough benefits from just increasing the single runs while keeping the doubles in but at the same length?