I am new on here, I have been taking my mountain bike out the past couple of years especially during summer time and bike around 30-50 miles some days while averaging around 70-90 hours a week

I have a mountain bike from costco made for off road use and it kills my lower back

I want to take a 3-4 day weekend and bike around 250-350 miles soon, I know I will be needing a new bike for this and I am a college student that has just moved out on his own so I am on a tight budget of around $700 or less, what would any of you suggest?

Thank you for your help I really appreciate it

also if there is such a thing as a bike that makes it easier to bike on the road but that I can still take on mountain trails after being on the road for 30-50 miles, I would really like to know about it

Thanks again, all help appreciated