To expand on the answer given by Ankit:
of late = recently
had fallen on evil days = had been having problems, or difficulties. I would not consider it an idiom myself, but perhaps it was a more common usage in the author's era.
To re-word the sentence in an attempt to make its meaning clearer:
Mr. Jones had always been a hard master, but he had also always been a capable farmer until recently, when some problems developed that he was unable to resolve satisfactorily.
from my perspective (American dialect):
To "get going" means to take concrete actions to prepare to leave. It can also mean "to hurry up." So, for instance, if I have a lot to do today, and I am dawdling over my breakfast and drinking my second cup of coffee, someone might say "you should get going!" But they probably wouldn't say "you should go!"
So "I should get going" is something I'm more likely to use when I'm implying that I have somewhere to go and stuff to do. "I should go" I'm more likely to use when I should leave. (For instance: we are at your house, we just had an unpleasant conversation, and I feel unwelcome: "I should go." I am at your house for a dinner party and it's time for me to go to bed "I should get going.")
Summary: "I should get going" implies I "should" because I have somewhere else to be, whereas "I should go" implies I "should" because it's important that I not be here. This is subtle but becomes more strong when we change "I" to "you."
As far as "I should be going," it's something I hear in movies but I don't personally say; maybe it is more British, or maybe it is just old-fashioned.
Best Answer
"I'm going to be late" means "I know I have no chance of getting to my destination in time".
"I'm late" could mean that you are behind schedule in the timetable you had planned for your daily activities (also "I'm running late"). You might use it when you arrive ("I'm sorry I'm late"), but later on you would say "I was late" or "I arrived late".