I'm going to make an assumption here, that since you used the term "day visitor," that means you're staying off-site at a non-Disney hotel. If this is the case, then when you arrive at MK that morning, you should follow the directions to the MK parking lot and park there.
The MK lot is not actually right next to the park, however; it is on the other side of the Seven Seas Lagoon, which is why you have to ride a monorail or ferryboat to get to the park gates. The monorail/ferry station at the parking lot is called the Ticket and Transportation Center, or TTC.
After you park your car and either walk or take the parking lot tram up to the TTC, you will see that there are three monorail lines:
1) Express (runs from TTC to MK only, then back again)
2) Resorts (stops at Polynesian, Grand Floridian, Magic Kingdon park, and Contemporary, before returning to TTC)
3) Epcot (runs from TTC to Epcot only and back again)
To get from TTC to 1900 Park Fare, take the Resorts monorail loop and get off at Grand Floridian. Once inside the GF, go down to the first floor; if you have any trouble finding 1900 Park Fare, simply ask any Disney Cast Member, they'll direct you.
The monorail from TTC to GF takes about 15 minutes, with a stop along the way at Polynesian. You might also have to wait 10-15 minutes for the monorail to arrive, then board, before you actually get moving, so plan on arriving at the MK lot at least 45 minutes before your reservation at 1900 Park Fare; this will give you plenty of time to get from your car to the monorail, wait for the monorail to arrive, get on, ride to GF, and get to the restaurant podium.
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Part II
If I am wrong in my assumption, and you ARE styaing at a WDW resort, and used the term "day visitor" to mean that you are only a day visitor to GF, then your transportation will work a bit differently.
If you are staying at a WDW resort, you can take a bus directly from your resort to the Magic Kingdom gates. Once there, walk past the park gates and you will find the monorail station on the other side. Again, take the Resorts monorail line; it will make stops at Contemporary, TTC, anf Polynesian before arriving at GF. In this case, go to your resort' bus stop at least 90 minutes before your breakfast ADR, because you might have to wait up to 20 minutes for an MK bus to pick you up, then spend 10 minutes loading, then up to 10 minutes drive time to MK, then another 10-15 minutes waiting for the monorail to arrive, 5 minutes to load, and another 20 minutes to get around to GF.
These are worst-case times, of course; it's ususally quicker than that to get anywhere, but sometimes you just happen to catch the Disney Transportation System at just the wrong time and everything takes forever. It's kinda like having a 10-minute drive to work every day, IF you leave at 7:42, but if you leave at 7:50, you catch all the red lights and traffic, and it takes you 20 minutes to get there.
To avoid all of that, you can also take a taxi direct from your resort to GF. The lobby concierge can call one for you. From the time the taxi arrives to pick you up, it would only take you 15 minutes or less to get to GF, depending on which resort you're staying in.
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Part III
Once your breakfast at 1900 Park Fare is done, you simply get back on the monorail and ride it into MK.