Newbie looking for advice!

Clare:

From a distance perspective, that would be roughly 20km a day. A very reasonable pace for most people.

Ultreya,
Joe

As Joe says, it will average about 20km per day. About 273km in 14 days.
This is doable...but you need to be pretty fit to start out and do 20 miles every day for 14 days. Be sure to get in as much training with your pack as you can.
Good luck. Starting in Astorga makes pretty good sense with your time allowance.
Buen Camino

Ed

There is one bus from Barajas airport to Astorga on Sunday at 2359, and three or more Monday through Saturday, so you can go directly from the airport to Astorga.

0935
1345
2359

From the downtown bus station in Madrid, there are more.

Hello Clare,
I walked from Astorga to Sarria(about 150k) recently in six days. If I had been continuing to Santiago,
I would have completed that section in 5/6 days,all going well.
I flew into Santiago and then traveled by bus to Astorga via Lugo. I made one mistake in staying in Santiago overnight and then catching an early bus to Lugo and a connecting bus to Astorga.
Having arrived in Santiago on the flight at 15.30 I could have a caught a bus directly to Lugo. I could
then have a caught an early bus to Astorga giving me more time to walk to Rabanal.
There is a regular bus service to Lugo.Please visit http://www.empresafreire.com
The old city of Lugo, close to the bus station would be a nice place to stay.
I wish you goodluck on the trip. You will really enjoy the camino,a great experience.
Make sure you prepare well before you go with many 20/25 k walks including a bit of climbing.
Travel light with no more than 5/6 kilos, excluding water.
Buen Camino,
Irishwalker