Good treadmills have a lever that raises the front end to simulate running up hills, because running on level ground does not do much to strengthen your upper leg muscles. Running strengthens primarily your lower leg muscles. You stress your upper leg muscles significantly only when you run up hills. Each one percent increase in the elevation angle on your treadmill requires four percent more energy.
Serious runners train by running intervals. They run a short distance very fast, rest and then run very fast again. A typical workout could be to run four half-mile repeats, averaging two minutes each, with a slow one-eighth mile jog between each run. If you run a half mile on level ground in three minutes, you will run three minutes and eight seconds on a one percent incline, or eight seconds slower. Runners can expect slower times when they run up hills, but they will strengthen their upper legs.