Body position and tires affect speed big time, and are among the cheapest to implement correctly. Are you willing to put a pair of aerobars on there for your TT effort? Also, are you running fancy tires?
What was the wind direction during your effort down the straight road? What is the pavement quality like?
Riding 25 MPH solo is no easy feat and not attainable by non-trained athletes unless you're the next Lance. Once or twice a month I'll do a set of 2x20' intervals and hit around that speed (or slightly less) for 20 very difficult minutes at a time. Could I do that for an hour on my road bike in an all-out effort? Maybe, doubt it though.
For reference I'm 70 kg and have an FTP near the top of that range (295 w) and feel that I am a fairly "slippery" rider.
EDIT: Also remember that power requirements have a cubic relationship with respect to overcoming wind resistance on a bicycle. The difference in power (all else equal) to go between 24 and 25 MPH is much, much greater than the requirements to hit 23 from 22 MPH, for example.
As with anything else it takes practice and experimentation! I would encourage you to get out there more often and hammer your brains out for an hour trying out different things
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