Someone did a nice youtube video on exactly that- charging your macbook via usbc from the regular old style usb sockets. Works fine, and doesn't "drain your mac" as some people incorrectly guessed.
MacPro USB-C can be charged with old style USB-A power sources
I'm bucking the "gotta have 61 or 90 watts of power!" trend. That high capacity allows for charging the battery very quickly, even while using your Mac to the fullest. This is the worst possible case. The more common case is a forgotten charger, or being in an airplane seat with only a USB-A available.
Going further, I much prefer a sloooow trickle charge for my macpro. I sit at a desk all day. I don't want to quick sear charge to 100%, then slow cook the battery all day. Neither is great for the battery. The quick charge is stressful, as well as leaving it plugged in after charge.
I don't want to constantly plug and unplug my mac during the day to stay in the sweet lithium charge state. I really wish Apple would implement a charge level cutoff, like a Tesla car has. Set desired charge level to, say 70 or 80%, and the mac automatically stops charging when it hits that level. Drops to 40%? Start charging again.
So since Apple doesn't have that, I'll slowly trickle charge it. Ideally, just feeding it enough power to not use the battery. Charging my macpro off of a 5w or 12w charger iPhone/iPad charger should work nicely for this.