I'm replacing a shaft and trying to figure out the best way to refill the tube with grease. Is there an easier way than pumping the grease gun through the lower grease fitting for a long time before it fills up to the upper bearing?

Nope fill it up till you see the automatic greaser filing up, i'm assuming longtail shaft, when i had mine and put a new shaft in it i pumped up the shaft till i saw the greaser filling up, then i started the motor and let it run for a little while and let it move that grease around and then pumped some more grease into it filling the greaser back up.