So basically, the question is whether it's better to do a single 'steady state' speed/resistance workout, or to vary it with intervals, right?
Intervals. Always intervals. :)
Steady state has its place. If you're training for a run of more than one hour, you'll need the endurance of plodding along at a set speed continuously, and steady state training is an important part of that.
But in terms of calorie burn, fat loss, strength, adaption, and cardio fitness, doing intervals tends to beat a steady state.
The specific intensity of the intervals, eg which heart rate you get to and how long that takes, doesn't really matter. Cycles of "push - easier - push" are good for you. Make sure your 'push' is challenging for you personally, and your 'easier' is enough that you can recover. You'll find your push gets into higher and higher levels and speeds as you get used to it.
Also google "High intensity interval training". This would look like a warmup, followed by cycles of 20-30 seconds full out SPRINT, with 1-2 mins cooldown/recover, for up to 20 minutes. HIIT is a very tough workout and shouldn't be done more than once a week until you're adept at it.
Deb, in New Zealand