Hello, let me share my experience from this year. I am 47 but not unfit, but also not as fit as I have been as have had a foot issue for about six months impacting lots of daily routines. I do walk most days and enjoy it and wanted to get out hiking to build myself up for a through hike in 2018 or 2019 for the first time. I started with walking in my local area with my boots and building up weight in my backpack to get my body used to it.
Although a long six hour drive, I decided I wanted to walk in the Lake District, I got the four OS maps, but quickly found the AA walkers map covered what I wanted in just a single map. I got routes from a few magazines like Trail and Country Walking, played about in OS Maps online (free with Trail mag subscription or £11 of Tesco club points) making routes and asked advice on here and got a great response to get me going. First walk in the Lake District was Langdale Pikes and got me hooked and I've been back four times so far and might yet be more this year.
I got the Garmin GPSMap 64s when on offer at Go Outdoors, this is good for geocaching too. I use Viewranger on my iPhone quite a bit and now also use Gaia Gps website/App for planning.
The AA 50 walks book many have mentioned is also a good start. Do check Amazon in the New/Used as I have saved a lot buy buying new from mostly the BookRepository for both maps and books.
If you enjoy walking and the countryside, your fitness will increase quite quickly and its a great feeling and will inspire you to go further and higher.
Hope that helps some, it's a nutshell but I'd go on all day.
Welcome to the forum btw.
Nick