I haven't exercised in a month, so why does it feel more like a year?

I haven't exercised in a month, so why does it feel more like a year?  I worked out Monday and I went back a couple weeks in my workout training schedule and I reduced my run walk ratio. Monday about 1/3 of the way through my workout I felt really good.  I started thinking maybe I should have just picked up right where I left off. However, by the end of my workout I was dying. Tuesday my calfs hurt all day.  I was complaining Tuesday evening and my 14 year old offered to let me use his foam roller, aka torture device.  Now I have seen my boys use this and I have seen the look of agony on their face as they do it. This does not look like something that I want to do, so I politely declined.  Todays run was torture, my calfs burned and hurt the whole time. I was regretting not use the foam roller. I have committed to use it before my long run on Saturday.  My question is how could I regress so much in only a month?