I've been running for years and I still don't get that 'euphoric feeling' everyone talks about. Maybe it's overhyped. I do feel good after a run, but it's not like I'm walking on clouds or anything. You're probably right about finding that sweet spot, though. I'll try pushing myself harder and see what happens.
I've found that shorter, more intense intervals are better for VO2 max improvement. Try 6-8x400s at mile pace with 200m active recovery. It's a killer, but it'll get your heart rate up and push your limits. Also, make sure you're incorporating proper warm-ups and cool-downs to get the most out of your workouts.