I think it depends on how hard you are wiling to workout and your dedication level  I mean how many hours per day, your meal plan, sleeping time and much more things so according to my understanding you can do a considerable different in few months time if you do it right

No I think I should have done better in the past but what to do past is the past I cannot drag even with elephants

I think it always depend what you expect from your life and how do you see it because sometimes we loose our happiness by trying to gain more and more things even worthless or impossible to achieve so simplicity plays a big role for happiness

Try "GURU" a 2007 great Hindi movie act by Aishwarya Rai and Abhishek Bachchan. It contains with great songs and the story is inspirational which is written around the success of a businessmen 

That depends on each and every individual and how they see the world ? what do they expect in there life time ? How far they have achieved ? and many other things so there is no exact common answer to that question but you can decide for your self by analyzing your own life but not for everyone in one answer