Probably your lower hamstring! Try doing a 'glut-ham bridge' aka sky humps to strengthen and make sure to activate your gluts (like, clench your butt cheeks using only your gluts) before running so you're not letting you hams do too much of the work :)
For Switzerland this year a big portion of the races will have a very nice amount of climb, so you'll been strong, explosive legs.
A really simple workout that I do is:
Do the first exercise paired with one of the second ones 3 times(4-5min rest after each set). This workout isn't hard to do at all, and it can be done pretty much every day(best time to do it is after a workout). The purpose of it is to slowly build explosive muscle in your legs, then with running you can convert that into slightly more slow twitch fibers. Basically, with this and running, after a little while you'll be able to tackle any hill ;P Oh, hill sprints are nice too!
5 explosive jumps(with 15-20lb weights in your hands)(make sure to crunch into a ball then bring your knees high. You should land on a stool 3ft above the ground)
6x1 legged squat with 10lb weights in hand and your back leg resting on a stool 2 feet high, then you lower yourself and then go back up with your squat leg almost straight)(Do on both legs)
5xSquats with 35lb weight being held by both hands in the center of your body(flat back)
If you decide to try Jan Erik's exercises be careful and work up to it. I.e. do the whole thing with no weights and good form for a few weeks and then slowly add weights until you reach the amounts suggested (and then always with good form and control. Otherwise you are likely to accentuate any muscular imbalances rather than fix them.
And also try and fix the original problem at hand here and make sure the backs of your knees and your hamstrings are strong enough first before moving onto more quad based workouts, or you can make some muscles imbalances like Canadian said.
Thanks for the help everyone, and I'll make sure to start strengthening my hamstrings and knees. Eventually working up to Jan Erik's exercise and others