Late 40s is a great time to get fit if you are not already there. The less fit you are, the more pain you will feel as you get older and you are on the brink of being old like me. Struggling with the early/easy parts of getting certified is a wonderful warning sign and you should consider heeding it. You'll enjoy diving and everything else more if you can up your fitness even a little bit. Others have suggested you swim more, that is a great idea but fitness in general is a better target than just being comfortable in the water.