Sign of getting old

I was playing football on Wednesday night with Ronan and some of the guys from his work. Just an hour long game of 5-a-side and for the first time ever in my life I was completely out of breath and energy after about the first 20 mins. It has been a year since I had last played football but at the same time normally I have more energy and its normally after not pacing myself that the first 20 mins give me gip....but hey.....this time there I was back in goal for about 15 mins of the game...completely wrecked. And here I am 2 days later walking around like I have a broken leg or something.....completely suffering for it....the lactic acid is killing me. I'm going to keep this up for however long I can get a game because I can't leave it a year again....if I do I'll be close to death....