In all fairness I think choosing the length of regular races is going a little too far, since they're not very long anyway. Even when they are 10+ laps, who doesn't have the time to complete those? Very few people I imagine.
As for what you said about 'time-accelerated races' for some races in Endurance mode, that's pretty much what I was suggesting. I just phrased it a little differently.
Honestly I think the length of most of the main races are too
short. Very few races in real life are going to be two or three laps on a small track. Having such short races in GT5 means you can't pick a car that's competitive, you have to pick something slightly better than the AI so that you can pass them all on the last corner, if you want a good race that you can actually win. And that's just not how racing works.
As far as endurance races, driving for 24 hours non-stop is definitely unrealistic. However, if they let us switch to B-spec in GT6, they'd better have really good AI, because I'd hate to have my race ruined by the computer's poor driving. The alternative, driving in realistic stints then pausing or suspending the race 'til you're ready to drive again, is already possible, and I think works plenty well enough.
If you want total realism from the endurance races, you're going to need to get some friends who you consider good enough drivers and have them race with you. It would actually be really cool if you could do this with online friends and watch them, but that would have to be pretty complicated, and honestly pretty risky trusting a long race to GT's servers.
If you don't have "teammates" to help you do the races, you're going to have to compromise, either by letting B-spec help you, or doing it yourself with realistic breaks. Neither is perfect but they're both better than driving 24 hours non-stop. Apparently some people have done that though. Why? Beats me.