[a_girlsdreams]

First off, mono sucksssss. It can last for about 2 weeks or even longer than 6 months. If you had it last semester, there is a chance that its back. Usually mono stays with you. Yeah you "recover" from it for the time being, but its usually always there in your body.
If you still feel tired and weak, chances are you still have mono. If the steriods arent working or didnt work, you should see the doctor again and talk to him.
Running could keep you from fully getting over the illness. Its like, for example, if you are tired from being up allll night, your body needs allll day to recover. But instead of sleeping all day you only get 6 hours of rest, your body isnt fully recovered. Its kind of like the same thing. If you had it a few months ago and never allowed yourself about 2-6 weeks to recover, and still feel tired, then its probably still there and coming back worse. Depending on how bad mono is, sometimes its just tiredness, sometimes its more physical, doctors have patients out up to a few months from school or work.
If you are not feeling 100% go back to the doctor. But you are defiently going to need about a month of resting to recover.
Feel better and good luck.