Best way to help with cardio.

My stamina is absolutely tanked. I'm 31, not overweight but I wouldn't say I'm "fit", slightly rocking the dad bod. Went on a hike last week and this week, and when it comes to the almost straight uphills I am gassed immediately.

During the week I get an hour of PT Tue-Thurs. What are good workouts to do to help with this? Is it really just running? Intervals? Weightlifting + cardio?