Constructive Rest Pose
When I posted my 20 minute video of me lying on the floor, it had already been edited down from the 40 minutes that I had actually stayed there. It made me realize how far my practice has come, but also reminded me where I started: 5 minutes at a time.
So if lying there completely still for 15 minutes seems daunting, try it for 5 instead. This gives you time to make yourself comfortable but not so long that you fidget all the way through it. This particular pose can take several minutes for your body to relax enough to get the full benefit, relax being the key word. When 5 minutes becomes doable and you feel that you can begin to relax, add a little bit more time.