Picking up on a couple of points raised:
Extra days - my (3 year) railcard shows an expiry date five days after my 63rd birthday, so extra rime is currently being added to online applications.
Use before 60th birthday - you can't use the railcard before your 60th birthday. It's not stated in the Terms & Conditions, but here
https://www.senior-railcard.co.uk/using-your-railcard/are-you-eligible/ it states:
You're eligible for a Senior Railcard if you are 60 years old or over.
You can buy one up to two weeks before your 60th birthday if you buy online, but it’ll only be valid from your birthday onwards. If you buy from a station, you’ll have to be 60 or over on the day you do it.
That explicitly rules out any use for travel before you reach 60.
As I live in London, I'm also applying for a 60+ Oyster card. TfL appear to be more relaxed about validity, as here
https://tfl.gov.uk/fares/free-and-discounted-travel/60-plus-oyster-photocard?intcmp=54724 it states:
How to use
Once you get your 60+ London Oyster photocard, you can use it straight away.
TfL's Conditions of Carriage don't explicitly exclude travel before your 60th birthday, saying only this:
4.10 60+ London Oyster photocards are available to London residents aged 60 and older and not yet eligible for a Freedom Pass.
If you hold a 60+ London Oyster photocard you can travel free on bus, tram, Tube, DLR, London Overground, TfL Rail and some National Rail services at all times (see Note A2). You can also travel on most National Rail services in London from 0930 Monday to Friday and anytime on Saturdays, Sundays and public holidays. You cannot use your 60+ Oyster photocard to travel between West Drayton and Reading.
I've only got 9 days to go, so I'll bide my time before attempting any discounted travel.