Genetically speaking, you cannot change the form and type of muscles you have. If you have big ass slow twitch muscles in your thighs and hamstring area and you find yourself in a rut, BLAST the area for a few weeks, pay special attention to it. For instance I was benching 200 lbs. when I first started out, but was only curling 60 lbs. - this was when I first started out mind you. I hit my biceps with reverse curls, close fisted curls, standing curls, preacher bench curls, reverse preacher bench curls, and some concentric training (pushing and pulling immovable objects or weight you know you cannot hit but try anyway). After a few weeks my muscles responded to being totally ripped up, destroyed and pumped with tons of blood from every possible position - and started responding, I noticed a lot of growth.

If you're only doing 1-2 excercises on your thigh and hamstring area - then you need increase that to 4-5 excercises for the time being - just a few weeks and pay special attention - and for god's sakes stop doing more than 25 minutes of running or bike riding. Cardio burns fat, but it burns muscle too.

If you jog, try doing sprints instead of long distances - this will build explosiveness and stronger muscles - instead of endurance based muscles. Another good excercise is to sprint up a hill and jog down, sprint 50 yards up a hill and jog back down - you'll get better wind, better muscle mass and more explosion in your legs than a hour jog.