Sylv from my one almost successful attempt at it last year, on a dead flat course with no sharp corners (i.e Dunsfold) for me to keep away from a 4th Cat race on a road bike would require a steady power output of somewhere in the region of 310 to 320w. For the sake of comparison I reckon that 325w is necessary for me to break the club record on a good course on a good day and would produce a time of about 55minutes on a reasonable day on the Horsham course.
The difficulty is that is simply the power output to stay away once you're away. First you've got to break the elastic. If I simply sat at the front of the bunch doing 320w, a rider directly behind me would only need to do about 260w (if he was the same size as me) to keep up. That is equivalent to the difference between my 25-mile power output and 100-mile power output.
In order to ride a bunch off my wheel in a 4th cat race I reckon I've got to hit 900w as I go and maintain about 450w for the first 1 to 2 minutes to make the gap stick. If I do that I've gone hugely anaerobic and that bu**ers up my sustainable power output. When I did roughly that at half-distance at Dunsfold last year I got a good gap and then struggled to maintain an average power of about 305w which meant I got caught on the last lap just before the final corner.
It is possible to ride away from the bunch (I have seen it done) but you need to be able to produce that initial burst of power and then sustain quite a high power output having shocked your system getting away in the first place. That emphasises the main difference between time trialling and road racing (apart from the bike handling skills). A road race tends to involve going into the red numerous times in the race and then having periods of relative recovery whereas when you're timetrialling you want to be right on the edge of the red the whole way round without going into it.
George being 2mph faster doesn't bear thinking about