Standing desk helps a lot for me- mid 50's, run 60+ mpw, 3:10 marathon.
You get used to it- on many days, I don't sit at all. I don't really get tired of standing. And I don't get drowsy like I often do when I'm sitting. Sounds like a silly cliche or joke, but it's true, if you engage the glutes, you can stand a long time with minimal fatigue.
I work from home so when I do sit I always have my legs extended or else I just keep standing up. Sometimes it is nice to sit for an hour or so- but then when I stand up, I feel tight and creaky and think, "wow, I need to stop doing that!"
The standing desk has been a great match for an overall effort to strengthen the core, increase hip flexibility, and change to a better running form that doesn't reach and heel strike but instead keeps the feet closer to center of gravity. Making those changes is really hard, but a standing desk makes it a lot easier.