Buy a return of £13.50 on the day from the station or if you can't wait www.eastcoast.co.uk will send you one in advance.
You are talking about eight services an hour to get there, including a fast one that takes 20 minutes, so no problem; bit more trickier after the game, might need a cab.
Examples
London King's Cross 15.52 - Stevenage 16.53
London King's Cross 16.54 - Stevenage 17.36
London King's Cross 17.53 - Stevenage 18.13
Stevenage 22.17 - London King's Cross 22.56
Stevenage 22.29 - London King's Cross 23.25
Stebenage 23.59 - London King's Cross 00.42 (last service)
Directions to the ground
The nearest station to the ground is Stevenage which is about one mile away from the ground. Leave the station booking hall and turn left towards the town.
Take the stairs on the right before the bridge over the dual carriage way and head along the A602, Lytton Way.
At the roundabout which has the police station on the right, take the second exit into Six Hills Way.
At the next roundabout take the third exit (south) continuing along the A602, Monkswood Way, passing a large Asda store on your right.
After about 3/4 mile (McDonalds/Burger King etc will be on your right) you will arrive at a roundabout and the ground will be opposite you on the other side of the A602.
There's the station, the blue blob, in the middle of town.
There's the stadium, left hand square off the A602, and a mile away from the town centre!
Remember, it's in a Group Save area that can cut fares in half if four of you can travel together, and any various travelcards will make it even cheaper.
To find out more, go here:
www.daysoutguide.co.uk/groupsave.aspx
(Thanks to www.footballgroundguide.com for stadium directions.)