Having done all my running on the flats of Stratford upon Avon, doing a 4k run on the hills of North Wales was something of a shock to the system... To say the least!!
I had to walk four times as my lungs and legs muscles were about to kill me. I basically did two 2km loops, in different directions. The change in elevation was over 60m on each loop. Very challenging on the old glutes! Got my fastest km though (I doubt I could fall much faster!)
Walking is the done thing so no worries. You can often walk faster than you can run if the hill is very steep. As you do more hills the easier they get, and less stress on the calves -on the uphill that is
I tried running with my calves but ended up using my bum quite a bit... I'm not used to that. Hence the large amount of walking!
Practice makes perfect - as far as hills are concerned. The more often you run them, the easier they become.
I started with slopes and I thought running was so hard until I discovered flat ground on holiday in October. The hill running will build on your stamina and Stratford will seem so easy when you get back. You'll probably be even faster
I live half the year in France, in a very hilly area, I did C25K there, and the other half of the year in Cambridge which is flat as a pancake. To be honest I've found both types of running challenging, but in different ways. I'm beginning to think I prefer the hills, flat running is relentless, but hills use different muscles in rotation and you make up speed on the downs...I was surprised to find I am slower in Cambridge.
All flat here too and I know I'd be finding it so much harder if I had to run uphill.
Hills 
Just for the hill of it!
Hills, hills and more hills
The attack of killer treadmill
