My understanding is that the math formula used to get your FTP from a 20 minute test is supposed to be representative of what you should be able to do over an hour. But It doesn't seem right to me. The last FTP test I did on Zwift, on rollers with a power tap hub I averaged 330 watts for 20 minutes. But there is no way I could hold 300 for an hour. I've done a few A races on Zwift and I'm like dying at 3.5kg/w and I'm looking at everyone else around me at 4.5 and 5kg/w and then just watch them ride away in the virtual distance.
Perhaps doing a 20 minute test on the road would be more realistic IDK.