What bike are you riding? If an MTB, a road bike might help
Sounds like they are fitter than you. Can you keep up with them when riding in a group? How far is the group ride? Maybe you need to add some hills into your riding and up the mileage? How often do you do the 26 milesI am on a BH zaphire road bike and it just seems slow as most of the other riders in the club do average 15-16 mph
Minimise the time spent going slowly.
Watt meters should be used in conjunction with heart-rate monitors to improve watts per heartbeat
and you will also gain leg muscle strength .