I have been following this group for several weeks and have learned a lot. I
have been riding a MTB for many years for exercise and just bought a Trek
1200 to spice up my riding experience. It worked.

I am 50 years old and with my new bike am interested in setting a goal. I
have decided to attempt a century ride in late October. I had been riding
30-50 miles per week on the MTB. That has increased to 75 miles per week
since acquiring the new road bike. My questions a Does anybody have any
advice on how to train for this goal? and... What is a SAG in an organized
ride?

You're probably fit enough.

SAG= an unnecessary acronym. "sag wagons" are support vehicles that
assist riders with mechanical failures and/or take them back to the
start when they have to abandon. Some say it means Support And Gear,
but I have maintained that the capitalization of "sag" is just another
instance of acronymphomania. Again, I would be interested to hear
from old-timers as to when 'sag wagons' became "SAG wagons," since old
bike books & magazines that I read (circa 1980) still use the
uncapitalized version.


Agreed on the fitness level. Go for it!

Per Miriam Webster (www.m-w.com)...
Sag
1 : to droop, sink, or settle from or as if from pressure or loss of
tautness
2 a : to lose firmness, resiliency, or vigor spirits sagging from
overwork b : to decline especially from a thriving state
3 : DRIFT
4 : to fail to stimulate or retain interest

At least a couple of these would seem to fit someone who started the
ride and may not be able to complete it either from a mechanical
(bike), physical (injury) or mental (just plain pooped) reason.