60 min. of constant running.

So I am trying to get myself back into running shape but the last time I was able to do a constant run for a long period of time was before I tore my ACL and LCL both of which are in my knee. The reason why I am running is because I want to run Cross Country for my school, this will be the first year that they have it and I think that I would be able to help if I can get into shape fast enough. But my first run started out being a 60 min. run at about 75% speed, not knowing what my 100% speed was i took off and started to run. 25 min. later I am exhausted and slow down. I definitely have a lot of work to do with only about 2 and a half months before we start school time.