Going the wrong way?

I was late getting out of work and thought I was going to have to workout here at home again. Fortunately, ended up getting to the Y but it was feeling rough again. I think my body is rebelling against getting so much exercise after being lazy for so long. I did 4 minutes of running (admittedly at too high of a pace, danged young kid running next to me at 9.5mph!) and my HR was 170!!! So I walked for 2.5 minutes and ran 3, still in the high 160s so I walked 2 or 3 minutes and ran 1.5 and it was STILL high. Then I just started walking 2 and running one. It was consistently 160 or 161 at the end of each run but down to 143 by 2 minutes of walking. I did that 'til I'd been on 40 minutes total and did 5 minutes of cool down. I still ended up doing 2.87 miles in my 45 minutes, so not too bad.