Maybe if you tried doing 5 sets of 15?  That would be 75 for the day.  And you could do 20,15,20,15,20 - and that would be 90 for the day.  I am not going to say it is easy because it hurts, but I think that if you are able to spread them out and do them throughout the day, it gives your body a chance to recover, both in terms of joints but also in terms of overall tiredness that comes from adding extra exertion into your daily routine.