It's been a long time since I "trained" on the bici, but I think the principles are pretty much the same regardless of the sport...
I keep reading that to get the greatest benefit from intervals, you need to tailor the intensity and duration to your fitness level. Now, I'm pretty lackadaisical about my own interval workouts, but try doing some of the workouts you read about in the magazines and see if that helps you any.
Besides intervals, there are four other key components to your fitness program: tempo, skills, distance, and rest/recovery. For your tempo days, pick the pace you'd like to ride, and ride at that pace or just a little quicker, for a short ride - whatever that means to you, 5, 10, 20 miles, however long you can maintain the pace.
Skills - smooth pedal stroke, hill climbing, cornering, descending.
Distance - where you push the distance at an easy pace, just to get your body used to time in the saddle.
Recovery - at 40, you're getting to the age where you MAY need more than one recovery day a week. Try taking two (non-consecutive) days off and see if it helps. Cross-training days are NOT recovery days; you can do some stretching, walking, short easy riding, etc., but nothing intense.
Then there's nutrition ... a whole 'nother topic.
ETA ... 12 mph is a GOOD speed for the trail. Save anything faster for the road. /PSA