Could I put my favorite game on a flash drive and have it run properly? (Like 5+ hours at a time)
Because flash drives are as fast as SSD's (I think).
Not really. A flash drive will be slower than a normal HDD in most cases when running an application because you are limited by the USB transfer rate, not the memory on the drive.
Oh lol okay.
My flash drive runs at 30Mb/s which is slower than an HDD.
If you have a USB 3.0 drive and use a USB 3.0 slot (or 3.1, even better), you should reach comparable transfer rates to an HDD. Possibly even better than an HDD, especially with USB 3.1. Would be totally playable. The transfer rates are only going to matter when data is being read or written to disk, which for most games is during load scenes anyways.
I played WarCraft 3 comfortably off of an external USB 2.0 HDD for awhile. YMMV