As others already mentioned Julia's Travel was very easy to find. You will go directly to their office and not awaiting vans. When the group is ready you will walk down the street to your van. I recommend getting there 20-30 minutes early. there are other tours leaving at the same time. i would recommend getting the add-on meal, we didn't and regretted it because they made the tour guests a priority and we lost time ordering off the menu.

around 3 hours.

I thought there was plenty of time for both towns, not much free time in Avila but I felt there was enough time spent there. If you opt out of the lunch in Segovia , you have plenty of time to wander around.

La salida es a las 9 a.m. y ya a las 7 p.m estábamos de regreso en Madrid

Spain was one of the safest trips we been on. If you are traveling alone make some friends in the group and stay with the group so you don't miss your ride back.

yes, it is safe to travel alone. Monika

I’m not sure, we traveled independently. Sorry. I would google, day tours- Segovia/Avila.

Well, our tour guide took us into the Segovia Cathedral during the walk through historical Segovia from the Roman Aqueduct to Alcazar. We did not pay for the admission, apparently, the tour guide/company has prior arrangement with the Cathedral or its staff. It is a very quick tour lasting about 20-30 minutes.

when we went, it was with others (maybe 12 people total)