sorry only just seen this but seems that you answered you own question , have done Cricklade a few times, a good low key race with a fast course. Hope you enjoyed the race, I missed it this year but hope to return
don't accept that. there is a basic structure that I was given when I was suffering fatigue. Do you check your waking (this is v important) pulse rate. This is a very good indicator of if you are r un down/ fighting lurgy etc. If it is 4/5 beats above your normal then don't exercise. The exercise (easier to do something that can be controlled like static bike) , do you have a heart rate monitor. start off doing 30mins every other day at 120 beats per min. If after a month of this it has no affect on waking heart rate then increase to 130 beats. If you find that the increase has an affect of heart rate then drop back to lower rate until ready for increase work load . Follow this until you get to 160 beats heart rate. You just have to monitor your body and mood. I was in a real mess this worked for me, I have recovered and ran 6 marathons plus numerous halfs long distance swims and long middle Olympic sprint tris. It took me I reckon over a year. Part of this was mental , as my confidence was in tatters . This method was given to me by a well known sports doctor. You must believe you will get better and you will run halfs again. good luck and believe
I do not like wearing headphones in races, I believe it also nullifies the organisers insurance and is also not allowed as part of english athletics road running regulations. If it says no headphones then its no headphones of any make and racing on roads and many offroad races state that.