It's possible to fly EasyJet to Oviedo(Asturias) and then get bus up to Astorga.If you want to arrive Aturias and get to Astorga them same day then you have to get from the airport to Oviedo by bus and then get bus up to Astorga. You would get there into Astorga about 9pm.
There is a tab for English in the upper right hand corner of the web page. When we took the bus in min-June we bought the tickets at the bus terminal just before we departed. The bus was less than half full so I don't think you need to worry about advance booking in the off season.
I am planning on taking the bus from Madrid to Astorga myself a week from Sunday. I will let you know how it was. I got the same recommendations on one of my posts and checked the bus schedule. It looks like it is very easy to get from Madrid to Astorga.
Why is everybody in such a hurry? We flew in to Asturias, it's not that far by bus into Oviedo, stayed at a convenient hotel near the railway station, had a pleasant evening in Oviedo, caught the train first thing next morning, a smooth beautiful ride through the mountains to Leon, walked round the corner to the bus station and had lunch by the river, arrived in Astorga mid-afternoon with plenty of time to look around and settle in before starting the camino the next day. There's something to be said for putting a bit of space between your normal life and the camino, a bit of slow-down clearing time so you can be ready for the new experience.