He didnt look old at all, he looked very good, he was very fast and was out striking Dustin pretty easily. There was no issues with cardio or anything else, it was simply the leg kicks that destroyed his leg and Conor didnt know how to fight with 1 leg gone as thats probably never happened to him before. Once the leg was gone it was all downhill from there, he started to panic and started being sloppy and got finished, before the leg went he was as sharp as ever.
To me Conor has always been this archetype of a fighter in other games you could describe as a "glass cannon DPS", hes pretty much all offensive and his chin is not the best, his leg health is not the best, cardio is decent, but his offensive game is great and he can keep it standing vs 99% of fighters who arent wrestling gods.
I see Conor winning early vs Dustin and assuming the legs hold up either finishing or winning a UD, Conor is much faster than Dustin and has better boxing, just depends if Conor can hold up with all the extra like leg health/cardio.