The "Colts are so much better!" narrative is interesting considering that they had arguably the least impressive December of any AFC playoff team. Even if I ignore getting curb stomped in Dallas (game meant nothing, sat Hilton), they still were seriously outplayed by Cleveland, could do nothing against Houston and weren't very impressive against TN (though it was a season ender, so it probably shouldn't count either).
As for their much improved running game, this is the yardage by their leading rusher (per game) since December: 37, 60, 3, 35, 56, 63. The 60 and 56 came against two of the worst run defenses in the league, Cleveland and Cincy, and the first 37 was actually Luck. They only had one game that even approaches "impressive" and that is going for 99 on a very good Denver defense. Of course, Denver's offensive woes gave Indy 34 minutes and 11 drives to tally that yardage and Herron only went off for 2.7 ypc.
Run defense does appear improved, but leading up to the earlier game, Balt, Hou, Cincy, Pitt and NYG went off for 90, 136, 32, 117 and 98 respectively. It isn't like everyone was cracking 200 on them and now they've crept back to respectability.
Of course, there are some personnel improvements, with injured guys returning, but overall the playoff success looks far more like favorable match ups than development since November.