I've been reading Jack Daniels. Also I was advised that when training for the marathon, the 800m and below intervals weren't really specific.
I'm doing 6x1000m intervals.
Hal Higdon is worried that if you do a hard session, your next day has to be an easy day. If you are doing a long run, a shorter long run, a tempo run, where do you fit intervals? If you've not done intervals before and you go out too hard then you could be jeopardising other sessions.
Personally I'm doing Long run, intervals and my tempo is 10miles long, only 3-4 miles of it are tempo. Then I have a bunch of easy runs between.
You don't have to do the easy run. Even if you do, you probably don't need to do a whole hour. That's just what I'm doing for Marathon training. I do 3 of them a week.
The Tempo is supposed to be 20-25mins at half marathon race pace. So even a mile warm up, mile warm down and 3 at HMP would do.
Intervals, I like the 1000m. Again I'm marathon training but 400m and 800m would be good. 20-25mins at pace is what you're aiming for. So a 2min 800m would be 10reps with 2mins in between. Again 55 mins of running. But maybe just do 6 or 8 reps?
It all depends. I don't like to have a session shorter than an hour. So make up time with easy warm up, cool down.