3 mile campus run

We ran a 3.3 mile route during class. I was late so I did not meet the class. Basically I ran myself. I run 2 mile at average 10.5 min per mile pace for 2 miles without stop. After I stop running and start walking near lakeside dorm, I was not able to start to run more than 0.5 mile without walking. My Forerunnner 305 measure the route to be 3.5 mile instead of 3.3. It toke me 45 minutes. Basically 2 mile run and then 50% walk and 50% run the rest 1.5 mile. I am going to push myself finish the same route sometime next week within 40 minutes.