I know that it sounds like a cliche but just enjoy your course, then you'll get the maximum benefit from it. As to how difficult: it really depends on your ability & state of mind & just how good your instructors are.
Don't get phased because you have only been flying singles, I trained with people who had previous turbo prop experience (as I did) & made a right pigs ear of things & others who had about 200 hours total & breezed the course.
One tip: don't think about the entire course stretching out ahead of you when you start, literally focus on each stage at a time & treat it like building blocks; before you know it you will have completed your course & have a B737 300-900 rating endorsed in your licence. It's a fabulous aircraft to fly. Best of luck.
