I started to write lots of my own ideas (read well read guesses with no personal experience, I'm pretty fast twitch) on the subject. Then I decided Tom's post is enough to support the idea that a normal Tinman schedule is fine for a slow twitch runner until they are pretty good, say trying to move up on a division 1 team or qualify for the marathon trials.
From back in 2009:
Depletion of glycogen, which is the stored form of carbohydrate, is the key to stimulating growth of mitochondria in a type of muscle fiber. To maximally stimulate mitochondria growth in slow twitch fibers, most runners must reach an intensity between 80-90% of Vo2 max; above which further stimulation is limited. The reason stimulation above 80 to 90% is limited relates mostly to fatigue generated by phosphate leakage to the extra-cellular space around muscle fibers and/or acidosis. Both causes limit total duration of stimulation. Since slow twitch fiber are the biggest portion of most runners total fiber population, and since a portion of the energy used by slow twitch fibers is fat (fatty acid), it takes a lot of time (read, many km or miles) to deplete slow twitch fibers, normally, compared to fast twitch fibers, which deplete energy stores quickly.
Key:To maximize depletion in slow twitch fibers - and to a large degree fast intermediate fibers (called Type Ia), the maximum permissible dose of training must be at or near 80 to 90% of VO2 max. The paces for these are shown in my training charts, located on the homepage of The Run Zone, in the "Tinman Training Tools" file-folder. Those paces are called Tinman Tempo and CV pace.
A Tinman Tempo run of 45 minutes and a CV interval workout lasting 22.5 minutes depleted slow twitch fibers about the same. Either can be used, or a combination of the two. Add in a 10 to 15-minute warm up and the same for a cool down and you have done a great job of depleting your slow twitch fibers, thereby stimulating growth of mitochondria, capillaries, and substrate use. A simple 65-70 minute workout, including 45 minutes of Tinman Tempo running, will provide a similar effect as running for 2 hours at an EZ to Moderate pace. The same goes for a 65-70 minute workout including 22.5 minutes of CV intervals.
You don't have to run 13 miles in training to run a half-marathon. I once coached a 1:17 half-marathon runner who ran 30 miles per week and the longest run he ever did was 10 miles. He did a lot of 8-9 milers with CV or Tinman Tempo running in them, though, and striders for form, fluidity, and muscular coordination. He ran 3 times per week, did the eliptical 2 times per week, and swam once a week (the supplemental training was about 30 minutes per session, and mostly at an easy speed).