So that's about five hours from the station, to the top of Tal y fan via me druids circle and then back again.
I reckon you should be alright, so long as you dont keep stopping to admire the view, which is considerable.
It should be fairly straight forward, I did it mefelf once, from Bwlch y dduefan, (did I spell that right) up the mountain down to the circle and back again. You might find the path a bit elusive if there is one at all, but its not too far. I dont forsee any problems, just go for it, but then again on the other side of Tal y fan is Maen y Bardd and cerrig pryfaid do you intend to see them also.
That might be pushing it, time really flies up there.
Hope that was some help.

Or

Get the train to Crewe and i'll take you up