BMI means nothing. cycling that far is ridculous. go see a nutrionist.
100km on the weekend is not really ridiculous.
If it has been a while since you have done a lot of distance...it sounds like you need food. Did you eat on the ride? If you were frequently putting in that kind if mileage, it would probably be possible without anything but if it has been a while, you gotta get calories in.
if the ride takes 3.5 hours, you will burn a ton of calories--probably more than your legs have stored--and you need to start eating before you feel like you need it.