Don't know why they are making you run for two hours. Long distance running is different then tennis fitness. In tennis you have quick bursts of speed, then stop, then burst again. This is similar to basketball or football. Distance running is mostly aerobic, you are running slow for a long time nonstop. I ran the Chicago marathon last Fall in 4 and half hours, but I sometimes I still get tired easily when I play tennis. Since you only have a couple weeks, you will probably not be able to increase you aerobic fitness that much. Anyways, you can use the couch to 5k program and try. http://www.coolrunning.com/engine/2/2_3/181.shtml
. You can start at week 5 or 6, and do 5 of these workouts a week instead of 3 since you are young and need less recovery time. You will need 2 days of rest per week for recovery. I would not do any physical activity the day before tryout and make sure you drink plenty of water the entire week before the tryout so your body is well hydrated.