The sub-theme though, is our need for God's mercy, the reason we need nourishment lest we faint on the journey, or fall away into sin. The Epistle (from Romans) focuses on our death to sin so that we can now 'walk in the newness of life'. The Gradual and Alleluia reinforce this message, talking about God as our hope, and our refuge.
The theme of our need for continuing conversion and repentance - and hence God's mercy before we venture to accept his gift of himself to strengthen us - is made particularly poignant at Matins.
The reading is from 2 Samuel 1-16, which tells the salutary tale of King David being reproved by the prophet Nathan for arranging the death of Urriah the Hittite so that he could marry Urriah's wife, Bathsheba.
I think the story has a possible link to the feeding of the crowd too (although Dom Gueranger sees no real connection), because to make his point, Nathan uses a parable about taking a rich man stealing a poor man's one lamb to feast a visitor when he had many flocks of his own from which he could have chosen from. We too can can our nourishment in forbidden places rather than that offered by the Lord.
In any case, the story reminds us that even the Lord's most devoted servants can fall, and fall badly. But all is not lost provided we truly repent, as David did. After all, it is not whether or not we sin that determines whether we become a saint, it is our response to it. If our fall spurs us to greater efforts, if we seek forgiveness from God, our sins too can be forgiven, so that we can again worthily receive Our Lord.