I wouldnt stress about it too much, not many take the perfect route of taking vitamins three months prior and exercising etc to be in perfect shape.
I took folic acid tablets with the first two, I was taking pregnancy vitamins with this one but have just stopped as they were making me constipated plus making morning sickness worse and have gone back to just taking folic acid tablets - which are the only essential thing you need to take. Have never excercised but I think I get enough running around after my 2 year old.
If you are reasonably healthy, minimise the alcohol (cease if you know you could be pregnant), dont smoke then you are on your way. You can start taking folic acid tablets now or a pregnancy multivitamin if you wish.