Hello, as Martin has pointed out, the first thing that you need to do is try not to worry about this too much and certainly don't panic.
You will not be going to jail for using an out of date Railcard.
Some of the offices at the train operating companies and the agents that sometimes deal with these matters can be busy, so it can take 6 weeks or more for a letter to arrive at very busy times. Hopefully you will receive what is called a 'verification letter' much more quickly than that and it is always best to wait for the letter if you can as you can then be sure that you have the correct case reference number allocated to the report made by the member of staff who spoke to you. The letter will give a very brief outline of what has been reported and will give you an opportunity to put your explanation in writing before any decision is taken about what action might follow.
Did you give your correct address and the new one that you will soon be moving to ?
I have to say that although it is known that thermal printing can fade over time, Railcards that have been well looked after can generally be read, even when the print has become very feint and the information that is recorded on the magnetic strip on the back of the card can be read by the ticket machines. Putting another card over the dated portion of a railcard will trigger suspicion in rail staff doing ticket checks and if they were to discover the card was nearly 6 months out of date, as in this case, a report is virtually inevitable.
If you are likely to be leaving the UK within the next 3 weeks, you could call the customer relations department of the Train Operating Company (TOC), explain the problem and ask them to contact the revenue protection / prosecutions office to see if they can help. Make sure that you contact the correct rail company for the train on which you were travelling as there are a number of different operators that run services on the Newcastle - Durham route.
Did the inspector take the Railcard and any other rail tickets from you? It is especially important to try to recall whether there were any other rail tickets that have been purchased at a discount since January that may also have been held in the ticket wallet too.
If you have no previous warnings or ticket irregularity matters, the TOC may allow this to be settled without Court action and if you can show conclusively that there have been no other journeys during the past 5 months, they may be particularly lenient, but you will need to be entirely truthful and very fortunate for that to happen.
If you genuinely cannot get someone to monitor the post whilst you are away, it might be a good idea to provide the TOC with a mobile number and email address by which you can be contacted.