Segovia is really small. You can easily walk between all of the sights you list, although it is hilly. I would start with when you can get tickets to the Alcazar and the Cathedral and go from there. I don't recall the Alcazar taking very long; the cathedral will depend on if you are doing the tower tour (it's a climb, but amazing views). You can walk between the two in about 10-15 minutes, so you could schedule both in the afternoon easily. The aqueduct is another 10 minutes downhill from the cathedral. I would suggest perhaps taking a car from the train station to Plaza Azoquejo, see the aqueduct, have some coffee. Walk up to see the cathedral, find some lunch (we loved La Almuzara), tour the cathedral then head to the alcazar. If you have time, head back down to the plaza and get a car back to the station, or get one at the alcazar or Plaza Mayor (cathedral).