As I lay in bed trying to sleep I had a most extraordinary sensation – it was as if acid was coursing through my whole body, sporadically making me jump up and wonder what was going on.

By the time I had to drag myself to the pool I was already feeling very tired. However, I managed the swim and jumped on a gym Wattbike to complete the 40km cycle discipline. It wasn’t long before I suffered considerable discomfort and as I reached the last three kilometres I was feeling decidedly unwell.

I managed to complete the full 40km and go to change into dry clothes for a 10km run outside. It was one of my longest transitions as my energy levels were almost empty and I struggled to don several layers to keep me warm enough in the freezing temperature outside.

At the start of the run it was very difficult to build any speed, I have become accustomed to easing into it now – the change of clothing usually results in a drop in heart rate that is difficult to raise back to a tempo – but this was as gentle a beginning as I can remember on any day of the challenge.

It wasn’t until I had completed six kilometres that I felt more comfortable and the body began to flow more easily.

Perhaps it was the aftereffects of the cold I had last week, or I had eaten something that disagreed with me, but throughout the day I feared what could have caused me to burn from the inside.

I am sure I will get over this latest concern, having recovered from severe knee pain, extreme calf strains and last week’s terrible cold, I have been amazed at how quickly the body can get better.

Having focused on every fifth day, I afforded myself a Christmas pudding in celebration of reaching another mini-milestone. The next horizon is 75, which will be quite significant.

I am completing 100 Olympic-distance triathlons in 100 days to raise awareness and funds to build a fitting tribute to fallen police officers and staff at the National Memorial Arboretum.

As I do every day, I paid tribute to all who fell on this date in the past. We will never forget the sacrifice made by the following on January 8:

1877 – Police Constable William Coles – Somerset County Constabulary

1891 – Police Constable William Finlay – Royal Irish Constabulary

1921 – Police Constable Frederick Smyth – Royal Irish Constabulary

1938 – Inspector Simon Taylor – Dundee City Police

1949 – Police Constable Frank Skewis – Kent County Constabulary

1985 – Sergeant John Beckett – Northamptonshire Police

1997 – Police Constable Kenneth Brodie – Dumfries & Galloway Constabulary