What you practice, you become
Whatever skills you hone, and type of activities you do, determines your type of fitness capabilities
When it comes to becoming fit, some individuals get the idea that becoming fit generally, and get in shape, its just about running, doing push ups, lifting some weights. But when it comes to do certain physical tasks such as lifting a very heavy weight for a single maximal effort, or running for a very long distance, they find that they may not do as well as they thought they would even though they have been engaging in fitness training and exercise, and thought there will be a general carry over of fitness no matter what they do.
The reality is that fitness is very specific. Whatever you practice, you become good and more efficient at that skill or thing you practice, because your person adapts to the demands required from those particular tasks.
So if you're looking to improve your fitness, you need to ask yourself, for specifically what reasons do you want to improve your fitness for? If it's for a more practical purpose, such as being more capable to climb a mountain, hike long distances, and feel less tired doing daily tasks in general because your livelihood is that - then you will need to practice those things gradually, building up to that level of activity and intensity gradually, or get into an exercise routine that encompass specifically these areas that you need to get good at. This probably means that you will need to go for regular runs, hikes, stairs climbing, hill climbing, etc when you're not actually doing those specific activities. Doing things like heavy weight lifting does have a bit of carry over, but just relying on that alone will likely not benefit what you want to achieve that much since your person needs to get used to doing the things you want it to do.
Its pretty straightforward. Whatever you practice, you become. You become good at whatever you practice, and do, with enough time, exposure and consistency. So when you are planning that exercise or fitness training, think about what you really want, and ensure that your fitness training involves direct carry over to what you want to achieve, as much as possible. Eliminate the unnecessary.