That might depend on the video capture device. What specifically are you looking at? If you have the funds you might want to check out the Atomos Ninja Blade or the Blackmagic Design Video Assist devices. Both of those allow you to record to SD cards and also serve as a monitor while you are recording. You wouldn't have to be tethered to a computer if you use either one of those. Each is around $400. I checked and the X-T20 is compatible with the Atoms device because it has clean HDMI output. That simply means you won't have any menu overlays and will be able to record at full resolution.
What size card are you using if you only get 15 minutes? According to the specs for that camera you should be able to get close to 30...