The slowbies in my bike club ride 12-14 on group rides but the cyclists aren't in a structured paceline. There aren't a lot of people - usually less than 10. The most popular ride is on city streets where a slower/heavier rider can get really tired due to all the stops then accellerating to catch back up to cruising speed. The group skill level has them spaced out enough that there aren't even many drafting opportunities. However, ~somebody~ has to be the slowest; there is no map or cue sheet and the leader will stop the others at turns until everybody catches up. While most people climb and descend at their own pace, there is more accommodation on the flats to ride along slower riders.
What got me out of the 10mph beginner rut was being more consistent and riding more hills, and pushing my limits.
Consistency means riding every week and every month, or finding a suitable cross-training exercise. You'll see more progress when you ride more than 3 days a week but allow yourself at least 1 rest day.
Hills provide unstructured intervals. Change up your rides so one day includes some 5-second bumps you can power over; some 1-minute horrible climbs that make you see stars; or some 10-minute shallower climbs where you just set a rhythm and go steadily after it.
When I started I could only perform at a low percentage of my max HR. I'd be exhausted after a few minutes at 135bpm (72%). Improvements in conditioning allow me to comfortably ride several hours averaging 155bpm now (82%). My fastest one-hour ride I averaged over 90% with a few spikes to 97% and no it wasn't comfortable at all. It may take a while for your body to adapt.
Try different cadences and gearing, so you spin a little faster than usual in easier gears; or get into harder gears and slow cadence to develop strength. Learn how it feels to go fast on a downhill stretch. Learn how to climb while standing, and try to increase how long you can tolerate the extra exertion.
I use strava to measure my progress. There is one hard uphill that takes me 50 seconds on a good day (70 seconds on a bad day). My goal is to climb it in 49 seconds. When I make that goal I'll set another, like not being the slowest person on that climb. I find it very motivating.