Could be an outside chance you were fatigued for the 20K. Add in the humidity and you worked harder than you would if you had time to taper. Gutting it out is the way to go though - it's the mental toughness that really helps in the long ones. You should go into the half with confidence based on the weather conditions you had in the 20K. This is usually the time when your head starts to play havoc with goals - it's normal to think you should back off - but it will all come down to the day of the race. If the weather co-operates, you feel good in your first few miles and you're well rested, you'll have a good race. The thing is to stay focused on your goal. If you make it - great, if not, you'll come closer to it than if you decided to back off before even going in. If your body is holding up and your training is right on target - there's a good chance you'll make your goal. Since you still have 3 weeks to go, have you done the distance in training at race pace? Or the distance with 1/2 to 3/4 of it at race pace? It should be a good indicator too of your capabilities. If it takes another year - not too worry, it will be savoured all the more. Took me 13 years to do the pace I thought I could do - and that was in Boston last year. Since then, I've had 2 more marathon PBs. I'm now considering the last marathon - a training run and moving on. Actually feel terrific - recovered nicely and am thinking of a 1/2 marathon September 19th. That is - if the weather co-operates at all. Not sure I could handle another hot and/or humid day this year.