Some concrete suggestions as to how to approach the big task, now you've got your "home patch" sorted.
1. Look at a map and mentally put on the back burner all the far-flung areas that are realistically not going to be able to cover
on your own while you're still so young. That basically means anywhere that involves staying overnight - other than where you have convenient placed relations. Then start persuading the family that it would be an excellent idea to go on holiday to Northern Scotland / Pembrokeshire / Cornwall / Kent etc!!
2. Having narrowed yourself down to the practical (which still leaves you a
lot of stations) - do some research on what rover and ranger tickets cover what areas (look out particularly for ones with good child discounts - which you're not going to be able to benefit from for ever!).
3. Once you've settled on a vague area, spend time getting to know the timetables (
https://www.networkrail.co.uk/indus...timetable/electronic-national-rail-timetable/) and identify the "problem" stations in that area. [Not something that Merseyrail itself has any of!]. To take an example - the Greater Manchester area (a fairly likely next step!) has, off the top of my head, 4 awkward stations that have a (very) limited service - Clifton, Ardwick, Reddish South and Denton - at least two of which will test your determination to "get off and wait for the next train"! (If you'd been doing this yesterday, you'd have had a 12 hour wait at Clifton or would still be waiting at Ardwick as after the 07.15 arrival on Saturday morning, the next train won't call till 07.15 tomorrow. ) - then plan your day round the awkward bits.
4. Not usually an urban problem, but my most frustrating moments (so far) have been when I've planned a tight turnaround at some minor station, only to realise as the rear lights of the train disappear into the distance that the only way of getting across to the other platform is a level crossing and the barriers are staying down to let the train I want to catch approach. I've learnt to have a look at Streetview to check the risk of being stuck for an hour or two in the middle of nowhere.
In case you haven't got one already - here's a useful list of all the NR stations currently open:
(actually that's the 2025 list - there's presumably a 2026 one somewhere - or soon will be)
Personally, I'm including the London Underground (and DLR by association) , Tyne and Wear Metro, Manchester Metrolink as all of these serve things that are / were quite obviously "real" railway stations (and indeed I ticked off a couple of them when they still were), but that's purely personal preference. I also include stations on heritage railways.
Apologies if any or all of this was obvious....