Originally Posted by Drew Eckhardt

Ride less with groups and more by yourself. Group ride fitness and terrain interaction with the pack speed and your weight are unlikely to produce optimum stimulus or rest.

Ride more threshold intervals (95-100% of lactate heart rate which is about what you'll average over the last 20 minutes of an all-out 30 minute effort, or 90-100% of the power you could sustain for an hour). 10-20 minutes is good. You don't need much rest between. With some training you might ride 2x20 with 5 minutes between. You get faster by stressing systems needing improvement and being fresh enough to go hard for those efforts. 1 minute intervals for 10 minutes are not optimal for forcing adaptations relevant (more plentiful and powerful mitochondria) to 1-3 hour rides.

Increasing your threshold power also increases how fast you can go at lesser efforts and how long you can sustain them.

Also note heart rate based zones need to be calibrated against your physiology - there's too much variation between individuals for the %of age formulas to be valid. Chris Carmichael has a system built on a pair of 8 minute all-out efforts which are psychologically and logistically easier to accommodate than Friel's half hour time trial.

Ramping up total stress helps too where this can be intensity (you go from 2x10, to 3x10, to 2x20, etc. threshold intervals and end up doing that 2 days a week) and/or time based.

You also need to have less stressful (intensity^2 integrated over time) weeks and months.

Given limited time you need a training plan which focuses on high intensity because training stress is a function of intensity squared.

Consider a hypothetical 180 pound rider atop a 20 pound bike who can just manage 250W for an hour with with .4 m^2 Sd and .760 Cd per Gibertini and Grassi's paper and .004 coefficient of rolling resistance.

A 95% effort for him is 237W and 23 MPH solo on dead flat ground.

At a 15 MPH all-day endurance pace he's barely turning the pedals over at 80W which is a 32% effort.

To rack up the same training stress and endurance capacity he'd net from a 3x20 threshold workout totaling 1.5 hours with a warm-up, rest between intervals, and a little time to let the sweat evaporate riding at that pace he'd need to spend 9:15 to cover 138 miles.

Reading some books on cycling training would be wise. Popular selections are Friel's Cyclist's Training BIble and Carmichael's The Time Crunched Cyclist.