Manning has lost his first playoff game 8 times. He's won his first playoff game only 4 times. For whatever reason, Manning-led teams have struggled to win their first game in the post season. DEN can only face SD, KC, or IND and I think each of those teams has a decent chance to beat the Broncos. I would say DEN has a 50% chance of winning (when historically bye week teams have had a 75-80% chance of winning).
Unless that "whatever reason" is "Peyton Manning", I don't really see how the performance of Dwight Freeney, Bob Sanders, Reggie Wayne, and Tony Dungy (or worse, Edgerrin James, Marvin Harrison, Chad Bratzke, and Jim Mora) is particularly relevant to our expectations for the performance of Demaryius Thomas, Wes Welker, Dominique Rodgers-Cromartie, and John Fox.
If that "whatever reason" *IS* Peyton Manning, then it seems like now is a good time to point out that 4 of Manning's 8 "one-and-dones" came in games where his team held a lead with 40 seconds remaining in regulation, or that Manning's kicker has never made a game-winning or game-tying kick in the 4th quarter or overtime, and Manning's opponents have never missed one.
If you really think a Denver win is a 50/50 proposition, there will be plenty of opportunities for arbitrage for you, because I guarantee you that Vegas is going to disagree pretty vehemently.