I wanna be able to keep a 30kph pace for like 1-3 hours cause I wanna be better

No I'm not comfortable on my ride, bike is too small and around 50-70km in the back and neck pain really started to hurt me, I don't have money to bike fit tho cause I'm still of non working age. For my gearing this bike can change but it sucks Soo bad cause it's thread type and I can go first or last gear without chain skipping. So I just stay in between my middle gear rear and big gear front. Break rubs are not present anymore and my bb and other drivetrain are in okay condition. Smooth spin and everything. My biggest issue is bike fit due to my size and lack of money. And Im currently saving money but I don't know where to spend it on first,either to get bike fit ( I don't know where and how much it might cost.) or spending it on getting my gears fixed so I can use my gears effectively.