The main flaw in riding to an average speed is the tendancies for courses to be "out and back". In my 50+ years of riding TT'S, I've never had a race where the wind has been the same on both legs.
You can't really ride to an average mph to achieve a 5 hour 100. You will slow as the race progresses.
A rule of thumb I was told for estimating a 50 time, was to take your 25 time, double it and add 4 mins. For a 100, it would be double the 50 time, plus a factor, which I never worked out.
In my youth (1968), I rode and finished my one and only 100 TT and I had target times taped to my stem and a Smith's pocket watch on the bars. I was close to 5hrs, but hugely disappointed with my time of 5hrs and 10 seconds! I often think if I could have done each mile quicker by 1/10th of a second, I would have beaten the 5hrs and the 20mph target.
Similar to a spocco TT I did a couple of years ago. Averaged 34mph to the roundabout then just 19 back to the finish. I was one of the last riders to set off and felt like Filippo Ganna as I was wizzing along. Saw a rider coming back and thought : He’s going slow, poor sod must’ve punctured. Then spotted another rider going the same speed. Then it dawned on me : oh crap, that’s going to be me in a bit…..
The only time I've ever ridden 100 miles it took me all day, including a few cafe stops and fish 'n chips at the seaside.
Bill Norris (who founded SPOCO) was a friend of mine - A sad day when he passed. He was a great person with good values (much like Alan Bates).