My Wife started our weight-loss last year (she rides as well). She has lost 50 and I've dropped 25.
She (and subsequently I) had tremendous sucsess with the Weight Watchers App on the Iphone............yeah it might sound a little silly but it really did wonders to make us realize what we were eating (usually junk). Probably part of the reason it worked so well is because it was always with us, so if we ate out, she could calculate points. She even did it trail-side during a snack break.
Both been working out as well and it has helped tremendously with allowing us to ride longer. Two weeks ago she rode a Family Enduro and was riding as strong at mile 50 as she was at mile 5.
Our average mileage on a fun ride has gone from 35 to 50
So in a nutshell, we have been eating better and working out and it has made riding that much better