Mr. Trainor.
this is the weirdest thread I have seen... lately. Very strange, as an °µÍø½ûÇø, to see somebody claim that anything outside of the Central Valley is unsafe... ??? what?  did they give you their 'own' testimonial... or is it just from watching TV or reading the internet?? Â
First... if you are coming here from the USA ... register with the State Dept (easy... on line) that you are traveling here. They tend to have good and up-to-date information; via the Embassy here in CR about which areas to avoid. There is no such thing, whatsoever, called, "avoid the whole country... outside of the only Big city".  Usually they list bad neighborhoods in the San Jose area to avoid, as well as: Limon, Liberia, some rural areas (generally in the north where there are a lot of Nicaraguan illegals) and of course; some of the beach towns... anybody with a brain knows that beaches sometimes come with bars, parties, drugs and or prostitution... but that is the Exception here...  not the rule.
Rural Costa Rica is just as safe as, if not more than, rural America. You have to use the same, general, good sense that you would use anywhere.Â
Most important is YOU... if you appear to be a 'gringo' foreigner, not sure about your surroundings, and 'older' ... you could be an easy target... but only at night or in a very remote area ... like a country road where there's nobody around; no traffic; no houses or businesses; etc. It's just common sense.  Be visible, and be smart.
I live in a rural area here... village of 150 people. I do not live with bars on my windows, cameras, security fence, or razor wire because in my opinion, that isn't living...it's prison. In fact, i almost never lock my doors; the windows are open year-round; etc. I make it look like I have nothing worth taking... so come on in if you must!!  I make sure that I don't stand out as Foreign nor do I show off my money.... and that I have a good, friendly rapport with my neighbors. Â
The least safe thing about walking on a country road is getting hit by a car... duh!!! I'm guessing that would be the case almost anywhere on the planet ... ???
I'm guessing that some of those above are venting their unhappiness more than giving fact based advice. If you spend time reading threads on this forum or others... you'll find that most °µÍø½ûÇøs in Costa Rica say that there is no reason to even be in San Jose... except to shop; go to hospital, occasionally.
The most popular °µÍø½ûÇø places tend to be semi-rural; or rural.. but close to things they are interested in. People tend to select locations for Temperature/comfort level; reasonable proximity to amenities; healthcare, etc.; close or not close to other °µÍø½ûÇø concentrations; and activities. I like walking in the mountains; and sitting in the natural hot water thermals... so I live where those are close... but also with ample bus route access; and amenities within 30-40 minutes.  I made a 'needs' list before I came here.
Sir... if it was totally UNSAFE to live outside of the central valley... then I'd say that 70-80% of all foreigners living here are gambling with their very lives!!  'Mail66' didn't even finish their 'list'... and the grammar was questionable... so I would take that one with a grain of salt... ??
Read and consider what DaveandMarcia wrote... they tend to be good reliable commentators on this forum and don't ooze the kind of gobbeldy gook that has been tossed your way... just saying. I wasn't even going to read your 'question' ... but glad I did... you have been ill-served; to say the least.Â
I take buses all the time; so don't worry about that part of your planned adventure. Never an issue unless a flood washes away a road or bridge... but we call that 'Tuesday' here. In all my time on these various forums... i hear nothing but mostly 'good' things about the life here and how happy people are to come here and just 'exhale'.Â
My advice... you haven't been here.... You haven't spent time here, nor lived here yet. Maybe, just maybe... take baby steps when venturing out on 6-8 mile walks in a foreign country... especially in Central America, where there are poor people everywhere and sometimes, opportunism tends to follow that particular economic demographic. So... Stick to town; walk in circles; etc. The only way to feel comfortable about a place is to get to KNOW it. Tourists can be targets; so don't be the 'one' that strayed and made himself an easy one.
If you look at google earth for the areas you plan to visit... and there aren't a lot of 'roofs' ... then maybe... taking an 'independent' walk in that area, before knowing it, isn't a well-conceived idea. Would you do that in Mississippi? West Virginia?  Idaho?
You should be fine. Just use your head. Sometimes, I think that the minute the plane takes off for Costa Rica... that people's brains fall out... and they leave all they have ever learned about being evolved human beings lying on some runway. ???  I had a tourist couple ask me if there were supermarkets in CR?  I should have told them no and that they had to hunt for their food.
Have a good visit; drink some of our wonderful water; and Pura Vida.