I understand your feelings of guilt, but you know that your father isn't going to change. If the business isn't generating enough to pay your father and he's already working 80 hours a week, then how will it generate enough to support you? Would your father also expect you to work 80 hours a week? Even working 40 hours a week, how are you going to support yourself? Are you going to take a second job (which would have you working 80 hours a week or more, just not all of them with the family business)?
If you do decide to join the family business, will you be able to work with your father's micromanagement? He's going to be micromanaging you
, too, the same as he does the other employees, you know. You have ideas for improving the business and increasing its profitability, but will he let you implement them? And if not, can you hang on long enough until he's gone so that you can put these new ideas in place yourself? Does your father even intend to retire or does he plan to work there forever? (Often when these business founders are so invested in their "baby" -- the company they started -- they refuse to retire and end up working there until the day they die, which can be many, many years past the time when they should have let go...)
In making your decision, you have to weight your potential guilt against the pain of having to go to work every day with your father breathing over your shoulder, taking on everything for himself and preventing you from doing the things that you know are best for the business (or worse, forcing you do to things that you know are bad for the company).
In the end, you have to decide what's best for you. Perhaps you could work elsewhere for awhile, so you can see how other managers handle various tough situations and gain some management experience yourself without your father micromanaging your every move... then when your father is older and (maybe) more ready to hand over the reins of the business, you can come back and step into the family business, with the freedom to implement new policies that you believe will help grow the company?
It's a tough decision. Best of luck.