I took the Virgin East Coast train from Edinburgh to London Kings Cross in October. It was about five hours. Though after requesting a window seat on the left side of the train for the best view, I realized on boarding that I was at a table (set of four seats, facing each other), and I was sitting backwards! Not what I'd envisioned, but it turned out to be fine. If I had to do it again I would have looked at the seat map more carefully and gotten a conventional forward-facing seat.
The food aboard the train was decent (better than Amtrak food in the U.S., IMO). The conductors were pleasant and it was overall a good experience, though unremarkable. I'd seen such beautiful scenery in the Scottish Highlands the week before that everything from the train window paled in comparison.
I bought my advance ticket online, directly from Virgin, prior to leaving the U.S., and picked it up at the Edinburgh station from a ticket machine. It was a pretty easy process overall; I just needed to have the reservation number, and the same credit card I used to buy the ticket. The only other thing worth pointing out, which would have helped me to know, was that you need your ticket to exit the station in London, so keep it and have it handy.