The best predictor of speed on a long ride is your maximum aerobic power.

The best way to increase maximum aerobic power is to do aerobic and threshold training. There are two excellent workouts to improve aerobic power: tempo training and lactate threshold intervals:

Tempo rides: go out and ride with a heart rate range of about 76%-83% of your maximum. You can do these a couple of days a week, as long as you're not fatigued.

Lactate threshold intervals: repeated efforts of 15-20 minutes each, at a heart rate of 88%-93%. These are tough and should only be done once a week. You can build up the intensity and duration as your fitness improves.

The rest of your rides during the week should be endurance rides (65%-74%) and active recovery rides (< 65%). Split up the hard days with easiser days.