I recently had lunch with a friend from medical school. She was telling me about her running coach. This coach was an attorney who got fed up with law and decided to become a running coach. She now tells people like my friend how far to run every day. Only problem is, my friend isn't losing weight the way she wants. So she does her own thing, like cross-fit. "It's the only way I can lose weight" she tells me.
"Why not run like you do cross fit?" I asked
"You mean run fast?" she says
"Exactly." I reply
"Because I can't keep up the speed."
One of the best ways to get fast, lose weight and build endurance comes down to interval training. Too many people follow programs designed for training volume with no focus on training intensity. Of course doing too much of one without any of the other isn't good either.
Unfortunately, most intervals are based on distance.
800x4, 200x8, etc. As you get faster, your intervals are getting shorter. If you aren't fast enough, the intervals are too long and fatiguing, which limits recovery.
Intervals should be aimed at the skills of the fast twitch fibers they are trying to target. Fast twitch fibers perform very well for 4-5 minutes (Type IIA fibers) and for 20-30 seconds (Type IIB fibers.) Of course, there is a lot of variability in there, but these represent good interval times to start.
Shoot for a goal of 6-8 intervals with complete recovery at the 4-5 minute pace or the 30 second pace.