Training in the hills will get you good at climbing hills, and it's terrific stuff for building strength. But if you want to build endurance, time on the bike is essential. The problem with doing nothing but shortish, hard workouts is recovery - your body doesn't have the time to recover and adapt between sessions, so you start to get tired and, eventually, burn out.

At your present volumes I wouldn't worry too much about that. But from now on I'd recommend increasing the volume of low-intensity work, and keeping the high-intensity stuff at or below its current level. If you are interested enough to read a long article on the balance between low and high-intensity training in elite athletes, here's one. the tl;dr version is that they seem to do better when spending about 80% of their training time at low intensities and only 20% at or above threshold. But of course, their training volumes are high.