Does anyone have experience exercising while taking a stimulant?

I really enjoy running, and I've run quite often. A 7 minute mile used to be a very comfortable pace for me. I'm also in exceptional health.

HR = 52 BPM
BP = 120/75

Then I became depressed. It took me three weeks just to open my mail, but I still managed to work out because I love it so much. However, I started to struggle just to run a mile in 8:30. Eventually, I could hardly even run and I stopped. (Over the course of six months)

I finally went in to see a doctor, and I've been on 300mg Bupropion for about 6 weeks. My doctor said it really shouldn't affect me as long as I don't drink too much alcohol.

It worked on the depression. I feel normal. However, it did raise my HR up to 175 bpm. My doctor knows this wasn't concerned in the slightest.

Anyway, to my point. I went to work out for the first time in months, and I ran a mile in 6 minutes flat without becoming tired. I went a few days later, and I ran two miles in 12 minutes on a slight incline. If I actually chose to push myself, I'm certain I could break into the 4 minute range. (My previous best is 5:30)

This really isn't normal for me, and while I like that fact that i can record great times, I'm worried about its effects on my cardiovascular system. I"m actually afraid to work out like i used to.