Signed up for XC race series (10 races) now what?

I just signed up for 10 races in a series in my local area in Denmark. My fitness is not good but my motivation to work on it is! I ride a fatbike with 120mm front and a fully with (150/130), and I don't know which to use for racing yet - will have to do timed laps for that. What are your suggestions for a training plan? I currently ride 2 times a week. I have in the past year bought the mtbfitness.co.uk program, but I dont know if it will be the best choice for XC races.

What length/intensity of training rides should I do now? Any suggestions or tips is greatly appreciated.

Facts: - XCO type races 66 minutes, as many laps as possible. Lap distance varies between 4 and 7 kilometers. - There are age classes, as I am 30 I will probably have some tough competition. - Races start mid-April - Low amount of elevation in races, high amount of elevation in my local training area. (example from 21 km today is 414m elevation) - I intend to make a good effort and hopefully finish midpack even though I am riding a fatbike. - I am looking to train in the gym as well as on the bike. Intervals might be done in gym as well as daylight is scarce this time of year in the nordics.