There's no easy way to get faster, you just have to pedal harder. That involves training at a level that is outside your comfort zone.
Unterhausen has suggested 20-minute intervals and they will work. Another alternative is "tempo" rides. You ride for between one and two hours at about 85% of your threshold. If you don't use a HR monitor, take that as being a pace at which you are breathing hard enough to restrict talking to shortish sentences.