I thought I would respond to this since I used to be Presbyterian myself
First, take a look at the miracle of the multiplication of the loaves and fish in John 6, which just so happens to occur in the very same chapter where Jesus gives His discourse on the Eucharist, stating the necessity of having to eat His flesh and drink His blood in order to have eternal life. If Jesus can multiply bread and fish, surely He can multiply His flesh and blood, so He can offer it to us without “losing” any of His “body parts,” making it quite possible for Him to offer His Body and Blood while sitting with the Disciples at the Last Supper. (Not to mention the fact that He changed water into wine at the wedding at Cana, a prefiguring of His changing wine and bread into His Body and Blood).
Now take a look at Christ’s promise while giving the Great Commission, “And surely I am with you, always, to the very end of the age.” (Matt 28:20). This is fulfilled beautifully in the Eucharist. This doesn’t cause any problem with the Second Coming, because then Jesus will come in glory from heaven for all the world to see. In the Eucharist His glory is veiled and He is seen with the eyes of faith.