Heart Rates

Well I am back at my gym today. I havent been there in two weeks. I wanted to run 3k but was sadly dissapointed with myself. I dont understand, I've ran 30 minutes straight (at 4.5) a few times now. Today though I couldnt go more then 3 minutes (at 4.5) and I'd have to take a minute walk (3.5) to catch my breath and control my heart rate. This happend last time.

I hate it because I dont know when to STOP pushing myself. My heart rate was hitting 202 and I know thats SUPER high for me (my recommended range is like 120-170). I dont understand this. I know the most effective work outs are at 50-80% of your max heart rate....

so does that mean that when my heart rate is up that high, I'm not being as effective?

Should I be slowing down? If so, how will I ever increase my ability. If not, how do I keep going without collapsing.

So confused.