For the DNC convention, from the time she walked out on stage until she finally exited, it was some two hours total she was on her feet. Therefore, she should have no problem standing for a 90 minute debate.
Hillary appears to have publicly experienced more significant health problems since the DNC at the end of July. Her physical condition appears to be deteriorating over the past couple of months. Keeping an awareness of her physical and mental condition and attempting to minimize (or at least conceal) any health problems, while at the same time delivering the lies she has to supply to the moderator and the audience has got to be stressful.