I have a bike that I left there which should be mechanically sound, but questionable contact points. I don't know what the gearing is, I think low 60gi which is a bit lower than I use for my utility fix. Haven't been on a fixed gear for more than a couple hours at a time, but am fine for 6+hrs on geared bike. I already do ~85% of my riding on a fixed in hilly terrain, so not sure what to expect for riding through midwestern cornfields again after 25 yrs.
One piece of advice I got from a friend is to just get off the bike every few hours to break up the monotony on the legs.
Along those lines, on my geared rides, I use an rando bag. For this ride, I'm wondering if I should use bikepacking bags for food/extra water to encourage me to get off and stretch.