Circuit training, kettlebells, free weights, core training would all help in the gym
On the bike, do intervals, drop your saddle and ride flat out for anywhere between 30 seconds and 3-4 minutes at a time, rest, and repeat. Or get a wee loop and time yourself round it, and try and keep your laps times constant.
Also practise riding really smoothly, carrying as much speed as possible, loosening your grip off unless you are on a difficult section, etc.
There are two angles to approach it from – increase the amount of punishment that you can take before fatigue starts to kick in, and decrease the amount of effort you have to put in to ride at a given speed.