**The most important factor in determining mitochondrial density is training volume--as all the non-scientists have been telling you, get your miles in!**
Some physiology if you like:
The half life of mitochondria production is 1 week.
In otherwords, say you are completely sedentary, and increase your mileage from 0-20 miles the first week. By the end of that week you will gain 50% of the mitochondria you are going to get from 20 miles a week at that training intensity. If you run 20 miles the next week at that same intensity, by the end of the second week you will gain half of what you gained the first week, to bring your mitochondrial density to 75% (additional increase of 25% relative to pre-training levels) of the extra that you will gain by training 20 miles a week at that intensity. With the above cycle repeating for a few more weeks, the 3rd week will bring you to 87.5% (increase of 12.5%) the, 4th week will bring you to 93.75% (increase of 6.25%), the fifth week will bring you to 96.88% (increase of 3.125%), the 6th 98.44% (up 1.5625%) etc. (Yes the decimals are theoretical, but the relationship holds well.)
Now, say you realize the 20 miles a week will get you nowhere as a distance runner and decide to increase to 40 miles per week. By end of the first week at this new training level you will have gained 50% of the mitochondria that will come from this new load etc.
This continues until you reach ~120 minutes of running/day which for an elite marathoner if carried out 7 days a week at this average training load comes to 140 miles/week (6 minute pace).
The law of diminishing returns does apply here. At 60 minutes a day you have about 75% of what mitochondria you'll get. But, the positive is that if you are running only 60 minutes a day, you have a whole 25% increase available to you.
So, once more to make it clear, the increase is as such:
Week 1: 50% (up 50%)
Week 2: 75% (up 25%)
Week 3: 87.5% (up 12.5%)
Week 4: 93.75%(up 6.25%)
Week 5: 96.88 (up 3.125%)
Week 6: 98.44 (up 1.5625%)
As a caveat, intensity does play a role in determining which fibers get the mitochondrial gains. However IIa fibers are recruited fairly well at easy to moderate intensities. Beware there is also much more to distance running performance than mitochondrial density. An 800m runner doesn't need the mitochondrial density of a marathoner. However, if mitochondrial density is high, then tempo runs are faster, long intervals are faster which means the pumping capacity of the heart and transport abilities of the vascular system can be stress more. Thus increasing mitochondrial density is first on the list.
The science may be new to you, but this isn't anything you haven't been told before:)