It is not too hard to improve endurance: just put in enough hours doing your favourite endurance sports. Achieving a decent endurance level while maintaining strength is not so easy.
Why? Because the energy needed for both is from competing systems. For endurance you need long-duration exercises so muscles 'learn' to use oxygen optimally to burn fat. Long-duration exercise triggers a 'master enzyme' called AMPK (AMP-activated protein kinase) that will 'push on the gene-buttons' resulting in improved transport and uptake of fat and sugar into muscle and increase of mitochondria turning fat and glucose into ATP. All this leads to improved endurance capacity. Interestingly, the fastest way to activate this magic master enzyme AMPK is not by long-duration exercise, but with high intensity exercise.
To increase muscle strength, muscles need to grow, for which resistance exercise is needed. Just like how AMPK is the master enzyme for endurance, mTORC1 (mammalian target of rapamycin complex 1) is the 'master enzyme' for muscle growth.
While AMPK is triggered by endurance training, mTORC1 is triggered by resistance training. As a result mTORC1 will make muscles grow bigger by stimulating protein synthesis.
mTORC1 will be triggered best when you ingest protein or amino acids during your resistance exercise, even moreso when combined with carbohydrates, since insulin also influences mTORC1. The bad news is that activated AMPK blocks the activation of mTORC1.
How to get around this problem? The answer lies in knowing that AMPK is turned ON during endurance training and then rapidly turned off afterwards, while by contrast mTORC1 is only turned ON after your resistance training is finished and then needs to to remain active as long as possible, preferably also during the night when you sleep and grow new muscle.
If you can, train for endurance early in the day and for strength early in the evening, followed by a couple of carb- and protein-rich meals and a good night sleep, allowing for optimal recovery and muscle growth.