I’m a couple of weeks away from taking a week off for holiday. So just using TrainNow until I have bandwidth for a solid block.
Today I’m undecided between 90 and 60 mins endurance workout, TrainNow recommends an endurance workout but the intensity varies greatly depending on duration.
If i pick 90 minutes train now wants me to do Bays +1, a 6.8 endurance workout. If I pick 60 mins the intensity drops quite dramatically to something like Petit +1. a 4.0 workout or even lower. Picking 75 minutes also gives an easier workout, as does 105 minutes but at 90 minutes it seems pretty set on my doing a 6.8 
I did Sitkinak yesterday a level 6 threshold workout and rated very hard - couldn’t have done another 13 minutes block at FTP. So think prob need something easier today.
I looked through the library and there are certainly a lot of easier 90 minute workouts that could be recommended.
Feels to me that this is just a weird edge case bug rather than intended behaviour, given that all other durations are pretty much recommending easier workouts - which seem appropriate given training recently.
Cheers.