My DD was doing 40 mins naps when she transitioned to one nap. She is also an early waker. We were having issues with her sleep at the time, and she was OT from refusing naps all the time.
We worked on the early wakeup - many times she did not go back to sleep, but eventually it worked. We went in, told her it isn't time to get up yet, still sleepy time, go back to sleep. We would say it at nap times when she woke early too, and leave her there for a while (chatting on the monitor). Eventually it worked for both.
In the mean time, we did offer two naps whenever we thought she would take them, in order to combat the overtiredness.