Your vocal chords are like muscles...you've got to condition them, as well as your diaphragm and your breathing so you're not out of breath. I sing a lot in church and the annual Christmas cantata. Before I do that, a few months before, I have to get my voice and lungs ready. You literally have to get your vocals chords and body/lungs in shape...just like you would your legs and lungs for a triathlon.
I practice singing scales at different ranges, lightly at first, working my vocal chords and practicing breathing. For me, it comes with practice, drinking lots of water, and knowing when to stop before my chords get sore. (Straining your vocal chords is not fun and usually takes a week to recover.) I take a break in between days...allowing my vchords to rest. The next practice, I sing a little louder, and slower; holding the notes longer...seeing how long I can hold it.
Also, find a piece of music that has a lot of infrequent rest commas. Areas in the piece where you can take a breath. Annotated by a comma, or where there is a rest. Training your body/lungs to take big, deep breaths during the rests/commas will help condition your lungs. If you can do a difficult song that's fast, with very little opportunities to rest, the others will be a piece of cake.
I wish there was a better way...but when it comes down to it: it's just practice, conditioning, breathing and hydration. Hope this helps! Good luck!