Using Wi-Fi to do a migration is a disaster, this can take a week (no I am not kidding) or more to complete. Using an Ethernet cables I have found also very unreliable. The only sure way of doing it is connect the Time Machine backup or bootable clone backup from the old machine to the new machine via USB. You mentioned you don't have a EHD, this tells me you have NOT been backing up. You have been incredibly fortunate to date but sooner or later this will turn out to be a disaster, at some point operating without a backup will catch up with you and you will lose all of the data on your computer, don't laugh it happens every single day to lots of people!
First thing tomorrow get yourself not 1 but 2 EHD's, one to backup the MBP! You can use Mac Basics: Time Machine backs up your Mac - Apple Support to learn how easy it is. Once the MBP has completed its first TM backup which could take awhile because you have 220GB of data to backup. When that backup is complete, disconnect the TM backup from the MBP, then connect via USB to the iMac and run Migration Assistant. Once you have completed the migration then re-connect the MBP's TM backup to the MBP and let TM do regular backups! Then connect the other new EHD to the iMac and use it as a TM backup HD. This sounds difficult but it's really easy and EXTREMELY smart computing.
Don't scrimpt on EHDs, get yourself high quality backup drives. The reason being backup HD's can (and do) fail, most of them time when you need them the most! I recommend buy OWC (www.macsales.com) Mercury Elite Pro series of EHD. Many experienced users on these forums will agree with me on these points.
DO NOT buy 1 EHD and try to share it, for many reasons, first it won't reliably work and it's cheap and shortsighted. EHD's are relatively inexpensive, especially when you consider how valuable your data is and how much you invested in your new iMac Retina!
