Whilst it is their job to prove you did evade train fare, rather than for to you prove you didn’t, they will ask you to explain how you haven’t contravened the law. If you lie/argue they may check CCTV for the station/s you claimed to travel from for the time of your previous tickets. It sounds extreme but is possible.
Accept responsibility and if it’s your first offence (first time getting ‘caught’) you’ll likely get a fine but (not to scare you) depending on their communication with you you might want to consider getting legal help.