I run twice a week and can bang out 38 for 10KM and 1:25 for a half, (running background).
I supplement this with two sessions on the bike at the weekend, normally up to 2 hours.
This pattern of training translates very well to XC riding/racing up to around the two hour mark. After that it all falls apart, (relatively).
I need more time in my legs on the bike. Simple.
What type of fitness increase do you want? Power, endurance, intensity? Running can help with some of these but not others.