Is just running enough?

Running is enough, but here is the thing, eventually you will need to add weights to get faster at the running... So why not start early?

:huh:

At what point?

Distance runners don't lift weights to make them faster. They lift weights to prevent injury so they can train consistently and it is this consistent running that helps them to improve.

Sprinters are a different story.

I was talking about sprinting

For the kind of body I want, running is not enough. Further, I'm in my late 30s and am starting to think about things like bone denisty and if I did zero weight training, basically my entire upper half would get very little weight bearing exercise and that's not good for bone health.

First: OP asked about health and fitness, not about weightloss. Two different subjects.

To the OP: You can be fine with just running but it's not encouraged for a lot of reasons. Most of them to do with imbalance. Too much stress in the same way with no break throws your body out of whack. It also limits your running progress (core training in particular is key to enhancing your running). Finally, as you age, strength training better protects your body than running. If you don't like weights at the gym, try some body weight exercises.

It's just like with food - diversity is key to keeping your body adaptable and functioning optimally.