The type of bike you get really depends on what type of riding you want to do. Depending on the state of the cycle/walking track, you might want a hybrid*
bike (with front suspension and seat post suspension). But if the bike track is in good condition, maybe a flat bar road bike would be better. Suspension means that you lose a lot of effort you put into cycling.
But if you are wanting to eventually do longer rides, why not consider a road bike?**
These are soooo much easier to ride, less effort required, and if you are riding into a head wind (which I imagine you will encounter in a country area), you are able to get out of the wind - on a flat bar you are sitting higher and have more wind resisance. I ride my road bike 13km to work each day. Love it!
The following relates to my experiences buying road bikes.
As a *cough* mature rider, somewhat challenged in the height area (156 cm), I also had difficulty in getting a bike. Many of the shops would only suggest women specific designs. These are quite good, as they are smaller framed bikes and have smaller, more proportional items (narrower handlebars, shorter cranks). I just couldn't handle that the WSD bikes all seemed to have pastle colours.
I have ended up with two men's frames bikes: My first roadie was an XS Fuji Newest 3.0 (43 cm), the second one is an XS Giant TCR C2 (42 cm). I had smaller head stems put on them, have a 80 cm stem on both. I know other women my height who are unable to get comfortable on standard XS frames, and feel better on WSD XS frames. Maybe it just depends on the person.
I really suggest you try to get to a few bike shops and have a ride around on lots of bikes. It might be difficult, given your regional location, but a bike browsing day would do wonders for you (when I got my Giant TCR, my bike browsing day started at 8.00am & finished after 5!)
Also, if you are already reasonable fit, it won't take long for you to be able to do a 50 km ride. It will just be a matter of building up the different muscles, and getting accustomed to the position on the bike (ie getting your ar*e accustomed to riding - get good nicks
). I did my first 100 km ride 4 months after getting my first road bike.
Just because you are shorter does not necessarily mean you need to go custom built.
*Don't get a hybrid, don't get a hybrid, don't get a hybrid, I have an XS 14 cm Giant hybrid at home sitting in the shed going no-where
**Get a road bike, get a road bike, get a road bike