Are you wanting to extend it to switch to a 4hr EASY? Some babies just aren't ready to be awake for too long, an 1hr20m doesn't sound too bad really for her age. If her routine is pretty good and she settles easily for naps etc that is the main thing - if you get her into an overtired state you may start to have problems settling her. Equally, if you start to put her down before she shows tired signs and she starts to complain, that would be a good sign to keep her up longer. It is definitely best to do it slowly if you do want to extend her activity out, it's about trial and error to find the right amount of time (and then just when you get that figured out it'll change again :wink: ). Also, you'll probably find the first activity of the day after morning wake up will be the shortest of all the activities and will probably remain like that - sounds strange I know.
My dd goes through days where she wants to be held more than others, perhaps your dd's bored and needs a change of scenery or just wants extra cuddling. Is she ok if she's lying on her own on the floor with you sitting nearby talking to her instead of holding her?