In addition to the suggestions already noted, I would suggest going ahead and trying a few (couple, one!) push-ups with your toes on the step and your hands on the floor. These do indeed feel harder than standard push-ups with hands/feet on the floor, or hands on the step bench, because they're targeting the pecs higher up. And, when you feel really enterprising, try one or two with your feet on an even higher elevation, to hit the upper portion of the pecs as well, as well as recruiting the anterior deltoids more. If you start to insert more challenging variations in bits and pieces, you'll be able to develop the endurance you need for standard push-ups much faster.
Another thing you may want to try is varying the width of your hand placement. The wider your hand placement, the more the pecs are recruited; the narrower your hand placement, the more your triceps are recruited. A good mix of these will increase your endurance even more.
Push-ups are worth the effort, both for the pec / anterior delt / triceps work you get as well as the core stability work you get (not to mention static training for the thighs and calves!). Good luck!
A-jock
Love dem decline push-ups
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