The weather is rainy. Think of London or Ireland, something like that, but less cold. Insects are not a problem here, in northern Spain.
How far the beach is... it depends on the roads, maybe 2h on average but it can take much longer (5h maybe?). I live in Santiago de Compostela, there is a beach about 30 mins away.
There are hills almost everywhere, but not many mountains.
Galicia is a beautiful place and you'll have very pretty beaches but realize that you'll enjoy the coldest sea water in Spain in summer and the sea in winter is really wild.
It is very rainy and you can feel isolated and depressive, specially in winter and fall. Climate is better that in London or Berlin for sure; rural internet... well this a different question.
Some old people can be also very troublesome. Fire forests from cattle owners are a real treat (100 declared forest fires between the last saturday and monday only in Galicia).
You can have also problems with Hunters and poachers. It is forbidden for example to shoot towards a path or road or < 500m of a house, but a lot of hunters simply ignore the law all the time. People has been shooted whereas doing footing or over a bike, other have collected lost projectils falling in their garden or crashing against the windows, and three or four people are killed mistakenly by hunters each year. Is not common but can occur. If is hunting day better escape to the city or keep at the house.
Poison can be another big problem locally (dog owners be very afraid) so if you end with the wrong neighbour this can be a hell.
But this can be a heaven also. Food is fan-tas-tic, varied and cheap (specially if you do not have cultural problems with sea food). Galizian language can be confusing for non natives but is not a problem. Most people are really sweet by nature and will be glad to talk you in spanish anytime (please take in mind that this is not always guaranteed in isolated villages of the radicalized Catalunya or in the Basque Country at this moment).
Mosquitos are not a big problem normally, neither cockroaches. Wild boars and roe deers will be a problem for your garden and fruit trees (but the first is soo delicious and unavoidable, and the second is so preetty and easy to fix...). Badgers, Wolves and brown bears either avoid people at all cost unless harassed, or just ignore you and keep themselves at a safe distance. Is very uncommon (but supercool) to see them. The first is very common, the other two very rare.
How far the beach is... it depends on the roads, maybe 2h on average but it can take much longer (5h maybe?). I live in Santiago de Compostela, there is a beach about 30 mins away. There are hills almost everywhere, but not many mountains.
Map: https://goo.gl/hqklRv Wikipedia: https://en.wikipedia.org/wiki/Galicia_(Spain)