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!)