What do you say to “This year, I really want to”?

Ask about her progress or the part that scares her. It's something she's actively working on, so questions about it feel personal in a good way.

Should I give advice if I've done the same thing?

Not in the first text. Mention that you've done it if you want, then ask her something. Unasked advice reads as coaching.

It's nice but closed. Add a question so she has something to answer.

Run a half marathon. I've signed up. I'm scared. It's the thing I think about on every run.

I want replies that treat it like a real thing I'm doing, with a bit of fun. How's it going, am I on track, what's the hardest part.

I don't want a training plan from a stranger.

Three new girls today. Pick a prompt, send what you would really send, and see who replies and what she thought when she read it. Free, no download.

What do you say to “This year, I really want to”?: Ask about her progress or the part that scares her. It's something she's actively working on, so questions about it feel personal in a good way.
Should I give advice if I've done the same thing?: Not in the first text. Mention that you've done it if you want, then ask her something. Unasked advice reads as coaching.
Is encouragement enough?: It's nice but closed. Add a question so she has something to answer.