9 What is, “delay not”? Because many men say, it is a long time till Christ comes. What then: because we say, “delay not,” will He come before He has determined to come? What means this prayer, “delay not”? May not Your coming seem to me to be too long delayed. For to you it seems a long time, to God it seems not long, to whom a thousand years are one day, or the three hours of a watch. But if you shall not have had endurance, late for you it will be: and when to you it shall be late, you will be diverted from Him, and will be like those that were wearied in the desert, and hastened to ask of God the pleasant things which He was reserving for them in the Land; and when there were not given on their journey the pleasant things, whereby perchance they would have been corrupted, they murmured against God, and went back in heart unto Egypt: to that place whence in body they had been severed, in heart they went back.
Do not thou, then, so, do not so: fear the word of the Lord, saying, “Remember Lot's wife.” She too being on the way, but now delivered from the Sodomites, looked back; in the place where she looked back, there she remained: she became a statue of salt, in order to season you. For to you she has been given for an example, in order that you may have sense, may not stop infatuated on the way. Observe her stopping and pass on: observe her looking back, and do thou be reaching forth unto the things before, as Paul was. What is it, not to look back.
“Of the things behind forgetful,” he says. Therefore you follow, being called to the heavenly reward, whereof hereafter you will glory. For the same Apostle says, “There remains for me a crown of righteousness, which in that day the Lord, the just Judge, shall render to me.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)