Think of MM as a method of driving in Normal with the "switch to ICE when battery is depleted" trigger point at a bit higher level...
If you switch into MM with a full charge in a 2011/2012 Volt, the estimated ev range immediately drops by ~14 miles (the MM buffer), and everything from then on operates just like Normal mode... you start off driving on grid battery power, and once the ev range is gone, the car switches to ICE and continues on. Later model Volts will "grey out" the bottom battery bars when you switch to MM. Of course, each time you turn the motor off, you need to switch back into MM when you start the car again.
As you approach the end of your trip, switch back to Normal, regain the use of that ~14 ev miles, and it’s as if you drove the whole way in Normal (and the extra battery buffer will be available if you do need extra oomph when driving up steep hills - if not, nothing wasted, no difference from driving in Normal).
You might consider doing this during the long trip stretches that go though hills - make a habit of switching into MM as you start the day’s journey.
Hold mode will also work (my 2012 has no Hold), but if it’s likely you might forget to switch out of Normal after a stop along the way, you can’t recharge the battery to recapture the buffer by switching back into Hold after you’ve been driving down the road for a while and remember. You can if you switch back into MM.
Last year I drove from Oregon to Michigan and back using this method (not sure if the buffer was ever needed when crossing the Continental Divide). I also like to reset Trip B each time I fill the gas tank... provides an excellent display of my "on the fly" gas mileage (except for the initial setting, no electric miles are included in the total), and how much gas from the most recent filling has been used (I like to refill when 1 or 2 gallons are still remaining).
Note that if you drive long distances between fully recharging the battery, the usage screen display has limits... For my Gen 1 Volt, when the trip total reaches 1272 miles, the Total Miles display freezes at that number (total miles usually includes some electric miles, so this is the first of the three distance display numbers to hit 1272). When Gas Miles reaches 1272 miles, that number, too, stops increasing, although the trip MPG, lifetime MPG, and Gas Used continue to display correct numbers. I don’t think anyone has only partially recharged often enough to see if the electric miles since last Full Charge also freezes at 1272 miles. The entire screen does reset with a full charge. Some say the max display of 1272 miles might be a limitation of the binary representation of the metric equivalent of that distance.