I think GT6 is much better in career mode than was GT5. GT5 had that horrible exp system that required you to drive race after race after race over and over and over again just to unlock the next race. The seasonals helped a lot but the repeatable ones did not come around until well after I stopped playing.
The races in GT5 all allowed you to use RS tires and pretty much any car so they were way to easy, The B-Spec did at least have some tire restrictions but still you could overpower the AI cars with ease and let Bob win pretty much every race only a very few were even the tiniest bit hard.
GT6 scrapped the XP system and that alone makes it better, There is no separate B-Spec career which is also better. The races have restrictions on both tires and PP which is again better and also gives you a better idea what car you should run and what tires you should use to get a decent race on the first try.
I never made it to the 24 hour races in GT5 simply because I was sick of trying to get enough XP to unlock them and honestly they are usually quite boring anyway. I ran them in GT4 usually had the lead within 2-4 laps and from there on it was just a matter of driving around the track over and over and over again with no challenge at all other than trying to stay awake and the motivation of can I lap the AI more times than my friend did.
The 24 minutes races in GT6 may not take nearly as long but they are much more fun and you can always do them 60 times each if you like

That would be more challenging than a single 24 hour race and not nearly as boring.