If you haven't ridden for some time it might come as a bit of a shock to the system! Using your legs for cycling is quite different to walking and sitting on a saddle gets some getting used to. Don't worry, it'll all come right eventually, but it'll take time. Start off with a few miles and build up gradually. Living in your part is an advantage as you have some great countryside round about.
I'm 66 and ride vintage drop bar bikes and I'm comfortable with that, straight bars might suit you better. The only way is to try some out, if the dealer will let you. Find one who will 'fit' you for the bike. Wheelspin Cycles look good, this might suit
http://www.wheelspincycles.com/product/bikes/hybrid-bikes/trek-71-fx-1320010-2016/ Don't discount
Halfords but people on here have had varying experiences, it's a bit ' Darren is the bike guy but it's his day off today'. If the sales guy doesn't sound that clued up walk away. Their Subway might be worth a look.
Decathlon do have some good bikes but you might run into the Halfords problem. Dawes are at the lower end of the market but should suit you well enough, maybe something with a triple chainring. Don't bother with suspension as it just adds weight for little benefit. If the saddle is not comfortable don't worry you can swap it out. It may take some time to find the right one.
Good luck, if you feel you need further advise ask again on here.