Does anyone know how to find out whether the old are still public?
I'm new to this and thought I might try a level ish walk for a day and se where and how far one can go.
Registering with the "Railway Ramblers" website might give you the info you're looking for: - http://www.railwayramblers.org.uk/homepage/homepage.htm
They have an online Gazetteer of UK's disused railway lines accessible for walking, which, (QUOTE) "has been brought up to date and now includes all known railway paths up to 31st October 2016".
(You have to register with the site to get access to the info on that gazetteer).
An alternative to old railway lines would be the tracks that are about, some of which will get you further up & out without being to strenuous, like round some of the lakes e.g. Buttermere, up valleys e.g. Langstrath, to some Youth Hostels e.g. Skiddaw & Ennerdale, the off-road parts of cycle routes e.g. NCN 71 by Threlkeld Knotts.
One word of advice is to use the time you take more as a guide to what you can do rather than distance, if you haven't already tried plot 2 routes on this sites maps one on the level the other up a mountain of the same distance and look at the different Naismith times, which are a guide to moving time rather than walk time.