this ^^^
Given your low mileage going out for a 30 mile ride is most likely too far, your young and not overweight so try to build on your base mileage 50 miles a week, then 60,70 etc etc until you’ve got miles under your belt don’t worry about speed base miles are key even for veteran riders, I used to ride well over 10k miles a year and once I stopped lower back pain became an issue for me until I built my base back up.
good news is a you do long rides stop hurting (mostly) then you can start adding in faster and faster rides, if it’s possible where you live try and avoid to many hills until your base is established... did I mention base miles yet?
This year I've done 4 or 5 thirty mile rides. I did those averaging around 14-15mph and my back started to hurt probably 20 miles or so in? It would then be a case of standing on the descents and rocking my back to stretch it out as best as I could. My legs and cardio system are comfortable closer to the high 16s or low 17s though. If I start going slower than that well, it feels like I'm not pushing hard enough! I've done a lot of interval training this year, which is why my averages over shorter distances have improved significantly.
I completely understand what you're saying about base miles. I used to do 100k rides once a week (in 2019) at just 12mph, and my back didn't hurt half as much as it does nowadays. Then again I was sat bolt upright, giving no thought to aerodynamics and just focusing on making the distance, in any time no matter how slow it was.
Avoiding hills where I live is an impossibility I'm afraid
No matter which direction I go, 1000/ft elev gain per 10 miles is pretty standard for me. I live in West Wales so I hope that makes sense xD Also as weird as it sounds for a new rider I love hills a tonne haha, partly due to my weight but I just love the feeling of hill climbing, it's so satisfying. Gravity is a far more fun advisory than wind resistance in my opinion, or maybe it's just because I have a higher power to weight ratio but a lowish max power
Thank you for your encouragement, appreciate it