When you bought your ticket, you will have ticked a box to say that you have read (or at the very least, that you agree to abide by) the National Rail Conditions of Travel (NRCot). Of course, no one actually reads these, but by ticking the box you agree to be bound by them.
You can find the NRCoT at
https://www.nationalrail.co.uk/National Rail Conditions of Travel.pdf. Rather to my surprise, the NRCoT don't explicitly spell out that a ticket is only valid for one journey unless it's a special ticket such as a season ticket. But I think it's taken as read, and condition 11.3 is not helpful:
11.3. A return Ticket must be used in the correct sequence (you must use the outward portion of your return Ticket before you use the return portion). The outward portion of a return Ticket is no longer valid for travel once the return portion has been used.
You will not be in a strong position to argue that you did not know that you could only use a ticket once. Even if the railway (or the court) accept that it's not such common knowledge that you can only use a ticket once that it should have been spelt out, then for all but your first outward journey it will be argued that condition 11.3 blocks reuse of the ticket for outward trips - and since you didn't have a ticket you will have to pay for all those outward journeys (and just as a hint, that would recover virtually all the money that the railway had lost in that one of the many oddities of fares on British trains is that a return ticket normally only costs a little bit more than a single).
The advice that has been given in this thread is good. Wait for the railway to write to you. Don't ignore anything that they send (and if the address you gave the railway is one that you might move on from soon, then make sure that your post is redirected to your new address - see
https://www.royalmail.com/personal/receiving-mail/redirection) and if you are lucky they may agree to settle out of court. Follow the steps that
@Hadders has recommended and you have the best chance of resolving this matter without going to court.