Nutrition: eat carbs, drink water. Eat protien afterwards to help in muscle regeneration.
I would gradually start to increase your ride times. For basic endurance training you should ride at least an hour and a half at low to medium intensity.
Do you have clipless pedals?
They are a must for serious on-bike training.
Are you training for racing in the spring/summer?
If so, I would definately recommend some books to look at such as Joe Friel's Mountain Biker's Training Bible.
There is so much information available on training from so many different sources. It all depends on what your goals in mind are.