Welcome!!
You don't need to do anything before or at the start of the event, except turn up with your barcode and listen to the briefing. But if you do have any questions, ask someone in a hi-viz jacket - they're the volunteers and should be able to help.
In short, what you do is listen to briefing, go to the start, run. When you approach the finish, there will be a couple of volunteers with stopwatches, then a funnel and at the end of the funnel is another volunteer who will give you a finish token.
The finish token is not a souvenir! Do not take it away with you!!
Also make sure you stay in the same order in the funeral as the order you entered it, and don't duck out of the side of it. This is because the next but marries up the click of the stopwatch when you passed the line, the finish token (which gives your place) and you. Get the finish token out of step with the actual number of finishers and the results processor will curse you and all your descendants!
Take your finish token and get out your barcode and go to a barcode scanner (volunteer waving a smartphone about). They will scan your barcode, then your finish token, and then will either take your token or tell you where to drop it.
Then when everyone is finished, the stopwatch results and the scanning info get uploaded and married up and you get a text/email with your time.
If you don't want a tine, you can either go through the funnel, take your token and just drop it unscanned at the collection point. Or not enter the funnel at all (so timekeepers don't click you)
Yes it's fine to walk - my favourite volunteer position is tailwalker. All the best people are at the back.
They may ask people to line up at the start in order if expected finish time (it eases congestion at big events). If you're not sure, then just start towards the back at your first one and see how you get on. It's a run not a race!
(PS: sorry - too wordy - it's really very straightforward on the day!)