I think there needs to be a balance since a miserable runner is not a good runner (so if cutting out alcohol or giant brownie ice cream sundaes completely makes you miserable, it probably won't be good for your running) but for the most part, yes, the "little things" add up to make a strong, fast runner. One small thing on its own probably won't have a large impact, but focusing on lots of small things together can have a noticeable impact.
I try to pick at least one "little thing" every season. Ones I've done over the last year:
1. Sleeping more
2. Improving my diet significantly
3. Finding out I was iron deficient and supplementing accordingly
4. Seeing a sports psychologist (temporarily, to learn some stuff) and practicing mental training
5. Strength training more often, both for power and for injury prevention
6. Adding in a core routine
7. Making my life outside of running less stressful so that I could train at a higher level
8. Sticking my calves every day
9. Taking planned breaks and backing off when I need to (working on this one, it's really hard)
Next thing I'm going to do is get personal training from an elite runner who is also a personal trainer, so that I can personalize my strength/core stuff a bit more and use it toward my ultimate goal of also becoming an elite runner.