Let’s see what we can explain on the basis of what you’ve told us.
The bad news is that you’ve definitely broken one railway byelaw (see
https://www.gov.uk/government/publications/railway-byelaws/railway-byelaws)
And you may have broken another one:
But you might not have broken byelaw 18 depending on how things were at your starting station (see byelaw 18(3)) - that’s why there are people here who want more details about where you started your journey and so on.
At the risk of stating the obvious, there are broadly two things that the railway can do now: they can decide not to go any further, or they can pursue you to resolve matters. The only way you will know if the railway aren’t going to go any further is to wait: if the railway haven’t been in touch for maybe the next seven months, then you’ll be able to assume that they won’t be in touch.
But if they do want to take matters further, they will write to you. That letter will most probably say that they are considering prosecuting you, but actually what it will be will be a request for you to give further information.
@Hadders who often contributes to these threads has some excellent advice on what to do when you get this letter - see (for example)
https://www.railforums.co.uk/threads/inspectors-on-gtr-great-northern.223009/post-5336809.
Normally I wouldn’t add anything to this advice - but in your case you need to decide whether or not you were in breach of byelaw 18. And we’ll find it easier to help you that if you can tell us where you were travelling from, and when.
Just a couple of things to add: to deal with this, it’s important to stay in touch with the railway. If the address you gave to the railway wasn’t right, then it is pretty likely that this matter will go to court and you will get a fine - which will eventually have to be paid when the court’s debt collectors find you. You’ll also have to pay the debt collectors’ costs.
And why something like seven months before you can be certain that the railway won’t be in touch? That’s because if the railway want to take something like this to court, they have to tell the court within six months of the incident. And then the railway has to tell you - which could take a few weeks more. Six months plus a few weeks is about seven months.