Anyone know roughly how long it takes to receive a letter? I will update in here once I receive it.
In terms of what you will need to say to them in your written response it is easier for ppl to advice on that when you have the letter and see what you are being accused of.
As mentioned up thread buy a new Railcard now, even if you have no intention of setting foot on a train any time soon (assuming you are entitled to one). This shows you took action to 'do the right thing' albeit belatedly.
To prepare yourself I would start listing out for your own records (but not post here of course) every ticket you might have bought with a Railcard discount for the period when you did not have a legitimate, valid Railcard. This will be the info they will be getting from wherever you buy tickets form and will put to you as travel with fares evaded - unless of course you buy your tickets in cash at a ticket machine. You need to price up the journeys at the Anytime Single (the most expensive) fare as this will be the sort of fare they will argue you evaded, even if you travelled off peak.
If you can secure a settlement offer from them (ie not court action) then you will need to be in a position to pay the sum they request in full and immediately. So you may need to take steps to have that money available.
But your initial effort needs to be focussed on getting the railway to offer you and out of court settlement (they have no obligation to do this), in the reply you will need to write to them (see other threads for examples)
I assume you gave them a correct postal address where they can write to you over the coming weeks/months and that you will be able to get your post from that address during the time (ie not heading off for a 2 week holiday) - since you will have a very short time in which to reply once they write to you - 14 days from date letter sent IIRC.
Head back here for more advice as and when you need it as there will be people to help you get the best outcome you can (ar perhaps the 'least worst' outcome....)