Letná is the high, open ground above the Vltava, where the bridges line up one behind another and the Old Town roofs spread out beneath them. You do not need half a day for it.
Head for the viewpoint first, where ten or fifteen minutes is plenty. Take one photograph, then have one more look with the phone back in your pocket. The light is kinder late in the day, the morning is quieter, and you should expect some wind either way.
Then walk ten minutes into the park, where the noise of the centre falls away, before heading down to the river and into Josefov. You end up walking a loop without ever feeling that you planned one.
Once you are back on Maiselova, sit down and have something to eat. The restaurant is open every day from 11:30 to 22:00.