Golf trip endurance

Fellow eggheads - I’ve got my first “real” golf trip pegged for this summer. 5 days of Sand Valley, Lawsonia, and potentially Erin Hills. Growing up, I played enough to have a game until I graduated high school, but never competitively. The last ten years I’ve played a lot more and land between 15-25 rounds a year depending on life.

Last summer I walked 3 rounds in about 36 hours out at Pinehurst and Mid Pines. My swing started to abandon me a few different times on day 2, but with 9 left on the trip, I got an awful case of the lefts that I guessed was because my legs were shot. With my big trip this year (and a smaller 2.5 day one out to Aiken before it), I am really hoping to avoid my game vanishing for the trip.

What can I do to prep? Is it strength training with my legs and back? Something else? I’m in pretty solid shape for my mid 30s. I play a little tennis, run, and hit the stationary bike from to time.