Here we go, thanks to Antonio for bringing this to my attention...
Question: You get to the end of Hadd's Phase I and want to know what to do next.
Go and race a 5k (road or track, it don't matter). Give it an honest effort on a decent route (but don't get too anal finding the fastest 5k route on the planet, you'll race often).
After Phase I, we need to split you into either (and most usually) Phase IIa, which is aimed at getting you fit to race anything from 5k-HM. Or you could go into Phase IIb (in some cases, and only if certain conditions are met) which is Marathon-mode (only). Of course you can go into IIa and then into IIb later, which is the more normal procedure. It's not complicated, it just depends on where your immediate racing focus lies.
So, Phase IIa is going to be begin based on current ability at 5k, and within that phase you will race on a semi-regular basis to confirm and continually assess rate of improvement. I hate the all-eggs-in-one-basket philosophy of folks who want to run a marathon in six months and do nothing but training till they get there! I love to see folks racing. There's nothing like races for learning the honest truth. And I love the truth.
If you are an ST (as you believe) and have spent some serious time in Phase I, you will probably find that you have no zip to race over 5k. So you may not like the result of this initial 5k. The usual complaint at the finish line is; I could go round again, but not get faster! So don't be worried if this happens, it is just a by-product of the deep-aerobic work of Phase I. Coming out of base-training, no-one should expect to race well at 5k, but we gotta know where to start. Don't estimate a 5k time. Just get on a startline ...
If you are an ST, your performances at all other distances are going to be limited by your ability at 5k. You can train till the end of time at 70-90% HRmax, but without judicious insertion of faster interval training you risk becoming a one-pace, can't get out of 2nd gear, long slow distance, kinda runner (and no-one wants that).
Like this; your best HM pace is probably never going to be better than ~92-94% of your 5k PB pace (this applies to STs). Your best M performance pace is never going to be better than ~88-90% of your 5k PB pace (again, STs) ... So it should make good logical sense that you need to start and continually work on making that 5k PB as good as it can get!
Never forget that. An ST should always be working on improving 5k performance, because every time that happens, getting a knock-on improvement at all longer distances comes relatively easily for that type.
Therefore to begin, based on the result of your initial 5k, I would move you on to building up to sessions like these (copied from page 1 of Cabral & Hadd thread, which is worth a read ... if it ain't too biased of me to say so — see the link Antonio made up above to read that thread):
Take your initial 5k (if a road time, knock off 15 secs for an equivalent track 5k since you'll be doing most of your sessions on the track)
We need to get the runner into a condition where he/she can handle work sessions like the following:
6 x 800m at 104% of 5k pace with everything from equal time recovery (down to) 1:15-1:30 recovery.
5-6 x 1k at 5k pace with 200m jog recovery in 90 secs
3 x 3k at 95-96% of 5k pace with 800m jog recovery
2 x 5k at 93-94% of 5k pace with 800m jog recovery
8k at 92% of 5k pace
16-20k at 85-90% of 5k pace
Note that these are paces and distances to aim towards. These are not where you start from!
Get after it!