If your ride is in September, you have some time to get used to clipless pedals. They do make a huge difference, since you can power your bike both pushing and pulling.

They are relatively easy to install and remove, so you could always get a set, ride for a week or two, and if you feel like you are not going to be able to adjust in time for your ride, have them removed. You could also get a pedal wrench and remove them yourself - no bike shop trip required.

Good luck!