Gregory the Dialogist

Or; To bear the burden and heat of the day, is to be wearied through a life of long duration with the heats of the flesh. But it may be asked, How can they be said to murmur, when they are called to the kingdom of heaven? For none who murmurs shall receive the kingdom, and none who receives that can murmur. (ubi sup.) Or because the old fathers down to the Lord's coming, notwithstanding their righteous lives, were not brought to the kingdom, this murmur is theirs. …

Catena Aurea by Aquinas ↗

Thomas Aquinas

There is also another question. What does it mean that he says, who have borne the burden of the day and the heat? Because they did not bear more than the time they lived, and modern people likewise. What, then, is meant? A threefold answer is given. The first response is that hope deferred afflicts the soul. Some at the beginning of the world bore the burden, because they knew that their retribution was being deferred; therefore they are said to have borne the burden of the day. …

Commentary on Matthew ↗