Well....its sort of really really inefficient, since push-ups work the opposite muscles that pull-ups work, so you can do as many push-ups as you want, but it wont be a substitute for pullups. That's okay though, you may not have known.
There is an exercise you could do since you say you prefer a low intensity workout and its called the body-weight row: Lie underneath a table and grip the edge with your hands at wider than shoulder width, with the edge of the table over your chest. Then keeping your body straight, pull yourself towards your hands, until your chest touches the table. CAUTION: make sure that the table is secure and you wont flip it over by doing this, since that can really ruin your day. This exercise is easier in comparison to the pull-up, but may or may not present some difficulty to you (I dont know where you stand in terms of physical fitness).

But if you wanna really increase strength, then pull-ups are the way to go, since if its difficult you will make (more or less) rapid gains in strength. Unfortunately there isn't a easy or low intensity way to build strength within any sort of appreciable time frame, but if you have enough desire to, then continue with the pull-ups! It may be tough and high intensity now, but with time it will become easier and you will become stronger!

Good Luck!