I am a relative newbie, so this is just based on my experiences and experimentaion...Betty Cannondale said:
If I'm lucky, I get one off road ride in a week. Other than that, I ride lots of dirt roads, paved bike paths, and roads (all on my mtn bike b/c that's all I have) and I hit spinning class about 2 times per week. I usually do one hill climb (1-1.5 hours climbing) and one workout of hill repeats (short intervals w/ some recovery in between) a week. I do this mostly for fun and to stay in shape (I have only one race under my belt) but I find this type of training works great for me! I started riding more b/c I have an injury that is preventing me from running (my first and true love sport) but cycling has really kept me fit. I find that as long as I'm out on my bike a bunch all week, I can make progress and challenge myself with my weekend off-road rides.
I really am in no position to be advising anyone on riding since I'm new; I'm mostly posting b/c I am excited about the progress I've been making...maybe it would work for you too!