Why did Peter almost burn himself out after Pentecost?
Peter was consumed by expectancy that the King would set up His Kingdom soon, so he poured every moment into preaching to Israel that the crucified One was their King. If they believed, the King would return and bring the Kingdom with all its Old Testament promises.
Les explains that the twelve men were so expectant of the King setting up His Kingdom now that it filled every moment of their existence. That is why Peter almost burned himself out in the early days after Pentecost, preaching to Israel that the One they crucified was indeed their King. If they would believe, the King would return, and they could have the Kingdom and all the Old Testament promises of going to the Gentiles. But God in His foreknowledge knew they would reject it. The emotion driving them is seen in Peter's words in Matthew 19: 'Behold, we have forsaken all and followed thee.'