Saturday - still not right

I don't know how to describe this, but despite an easy week, I felt terrible still. It's not so much during the warm-up, it's once the speed comes. Although after the warm-up this morning I was having serious doubts about finishing. Started Parkrun, staying with the leaders for the first kilometer. This felt fine, but literally within a few hundred metres I was back to jogging. I just couldn't do anything about it. It wasn't even as if the pace was that tough! So I jogged back to the start/finish line, grabbed my jacket and ran home. A pretty disappointing way to start the weekend. I don't know what to do from here? I'm just praying that I haven't relapsed into what I went through early this year!