If you want to ride for 1 hour at 25 mph then you’ve got to start by riding at 25 mph!
Once you have decided on your target speed for your race then you need to practice riding at that pace but (importantly) in your TT position (when the roads are safe to do so). Where possible, always train on the road not the turbo.
If you want to ride a 25-mile TT in an hour, find a flat road and ride at 25 mph until you can’t sustain an average of 25 mph any longer. Now ride that road in the other direction to check that your effort wasn’t gradient/wind assisted!
At each subsequent session, increase the duration that you can sustain 25 mph for.
Do this session twice a week. You’ll be under the hour in no time J
There is scientific proof that training just above and just below your lactate threshold (an intensity where you begin to tip into an anaerobic state if you go any harder, normally found by measuring your average heart rate over a 20 minute time trial) can improve your time-trialling but in my opinion, as long as you have a strong aerobic base, training harder and faster makes you go faster!