@Trainfare101. Welcome to the forum.
You can probably expect to receive a letter from the train company's investigation team in the next few weeks advising that they are considering prosecuting you and asking for your explanation of what you've done.
Problematic thing is that if you've inappropriately been claiming a railcard discount for some time, and then travelled without an accompanying valid, in date, railcard, you'll be treated as not having had a valid ticket for each and every such occasion.
The train company will be wanting you to pay them all these unpaid back fares, and so the total amount due could quickly add up. They'll probably ask you for a low three figure sum, on top, towards their admin and investigation expenses.
How many times do you reckon you may have travelled after the railcard expired, and what would the typical journey have been?
Easy enough for them to check your ticket purchasing history if you've bought your tickets online using the likes of Trainline.
Could be expensive, but if you properly co-operate with the train company's investigation team, you should avoid being taken to court, where, from what you've told us, there's every chance you'd be found guilty.