This method will get you all 10 legacy score-related trophies in 30-60 minutes. It's the same method that's been used since Madden added the "sim 10 years" option, so I take no credit for this solution!
(1) From the main menu, hit
to go to the Customize menu. Select Share & Manage Files, then select Download Community Files.
(2) Hit
to go to Roster, sort by number of downloads, and download the file named MEASYTROPHIES by M-Easy. This was the first roster I found that did the trick.
(3) Go back to Share & Manage Files, go to Load and Delete Files, and load that roster.
(4) Head back to the main menu, go to Connected Franchise. Select offline, then hit Start a New Franchise. Select Use Active Roster.
(5) Scroll right to the San Francisco 49ers. If you loaded the roster right, the top 3 players will all be 99 overall. Select them as your team.
(6) Select Change Role. Select Coach, then Create Coach. Set your style to Team Builder, then create your coach (hit Accept & Continue). You don't need to change anything else about him.
(7) Go to Settings. Set Salary Cap to OFF, make it All-Madden difficulty, Fill Roster to ON, Re-Sign Players to ON, Progress Players to ON, Sign Offseason Free Agents to ON.
(8) Back out with CIRCLE, then hit Start Playing Regular Season.
(9) Select Advance Week, hit down, then left, then
to sim 10 years. Simming takes a while, so walk away and check back later. If you sit there and watch the trophies (slowly) pop, you can cancel the sim when you see the Peyton Manning Legacy Award unlock.
NOTE: To check your progress at any time, hit
to cancel the sim. Then, go to Manage, then My Coach, then Coach Goals. On this screen, you'll clearly see your legacy score! To get all 10 trophies, you need to exceed 20,000.