Hi
I'm a Guard ('Train Manager', it's all the same thing!) with a long distance operator, similar to what you'll be doing if you're successful. The role of Guard is often thought of as simply being the miserable bloke who goes through clipping tickets. It is, in fact, far more technical than that, and doing the revenue stuff is only one aspect of it - and not the most important one! If you get the job, you can expect about 3 -6 months of training, coaching and shadowing before eventually going out there by yourself. You'll do a lot of exams, which will include the dreaded 'Rules', Personal Track Safety, and then your traction technical stuff (the trains themselves) and finally your route learning exams, one for each part of each route you'll work over. So as you can see, it's rather involved, and that's why Guards sometimes get a bit arsey when people suggest it's just about doing tickets!

If you are offered the job you'll be sent for a safety critical role medical, which tests to specific standards things like your eyesight, colour vision, hearing, heart function etc as well as a full drug and alcohol screening.
As far as route knowledge goes, you'll be sent out to 'learn' each route you'll work trains over. Essentially, if something goes wrong you are one of the first lines of defence, along with your Driver, and it may fall to you to make the emergency calls and manage the situation. Equally you may see a passing train with something dangerous about it, or an obstruction across an adjacent line, or something/someone frying on the overhead wires. As a Guard you are trained in how to contact the signalman and tell him what he needs to know. That includes the location of the problem and exactly which of the lines (tracks) is affected. So you will go out and learn the names of each railway line (for example the Up and Down Main, Up and Down Relief, etc). You'll need to learn the name of every junction you pass over, any road level crossings, speed limits, whether the lines have electrified wires or rails, names of tunnels and viaducts, and which signal box is in charge of which area, and its boundaries. You'll also need to be able to identify roughly where you are at any given time, and in particular when you're approaching a station.
In stations, you will be responsible for unlocking and locking the train doors, as well as dispatching the train (telling the Driver it is safe to go, essentially), and to do that you need to know things about the station and each platform. Is there a signal for example. If there is, you need to check it isn't red, as if you try and dispatch when it is you'll be in trouble! Will the whole train fit in the platform, and if not how would you safely unlock only the carriages which do fit. Are there staff on the platform to tell you when it's time to go, or is it 'self dispatch'. There are many things you'll need to know, but you soon pick it all up.
'Traction' - i.e. trains - will involve learning all about the areas of each type of train (just the two for East Coast I would presume) that you will be working with. The routine things will be door operation, where all the emergency kit is and how to reset the alarms etc, and there will also be fault-finding. For example if a door fails during a station stop, refuses to open or close, you will be the person expected to know how to safely isolate and secure it. That can be a little stressful if it's half past five on a friday evening and the train has 600 impatient people on it, and other trains are stacking up behind you! There will likely be some climbing about on the dirty bits, learning where various isloating valves are and suchlike.
Personal Track Safety (PTS) is a fairly simple bit of training regarding safe conduct if you have to go down on or near the railway line itself. That could be on a depot or siding, or on the main line somewhere in an emergency or if there's a problem with your train. When you pass it you'll be issued with your PTS card, which every Guard must have, along with a SCWID (Safety Critical Work ID) card, which you also need.
The 'Rules' is a big exam, taking a full day or two with several hundred questions. The railway has a 'Rulebook' which contains the governing regulations for absolutely everything. There are lots of different modules and you will need to be familiar with those which apply to your job. Some of it is rather obscure, some more simple. You'll be fully trained in it all beforehand, but you need to absorb it all and it can be rather brain-frying! That will then crop up every two years for the rest of your career, deep joy!
Don't let that put you off, but do bear in mind that being a Guard is first and foremost about taking safe charge of your train and the people on and around it. If you show an understanding of that, you will be ahead of already. Best of luck!