I actually really like the idea of interval/tempo work embedded in continuous runs.
It's all about running fast while remaining relaxed. I find that when I do intervals from a rest state, I have trouble getting into a relaxed rhythm that feels like a good race tempo. For instance, when doing an XC workout like 4x2km, each 2km interval feels like a mini (slow) 2km race rather than part of a 8km race effort. I find these types of workouts have a negative effect on my race performance; and additionally, my university XC teams performances always seemed to decline from the first September meet after doing these types of workouts.
On the other hand, I would do continuous "workouts" in summer, which got me in great shape while never really feeling too worn out. Usually I would run 60 to 70min, starting out at my usual 6:30-6:45 per mile, but after 10-15 minutes (once I was feeling comfortable) I would gradually increase the pace (based purely on how I felt, and continuing the relaxed rhythm throughout) until I was at about 5km race pace (about 5:00/mi +/-) and would hold that for about as long as I could while keeping my relaxed rhythm. Once I pretty much hit the limit, I would hold a moderate pace (maybe 5:45-6:00/mile) for a couple minutes before slowing to an 7:30-8min/mile jog to recover.
So 60-70min continuous would take the form of:
10-15min at normal run pace (6:30-6:45ish)
5-10min gradual build-up to tempo pace (6:30 down to 4:50-5:20ish)
5-10min at tempo pace (4:50-5:20ish)
2-5min at moderate pace (5:45-6:00ish) ***the hardest part
5-8min at recovery pace (7:30-8:00ish)
25-40min at normal run pace (6:30-6:45)
I would also often finish off runs by running 3-5 laps of the track (and continuously from the rest of the run) in which I would run 250m at the run pace followed by a 150m build-up stride each lap. Often both parts would increase in pace in successive laps, with the 250m segments nearing tempo pace, and the 150 build-ups hitting sprint pace. I found these were really good for developing a kick, while also maintaining 800m-type foot-speed. I have an training log entry from Dec '05:
62min run with build-up tempo, finishing on the track with 2000m of 250, 150 - splits 82, 81, 75, 69, 69