Interval Training is Hard

Today's run: 2.68 km (1.67 miles) in 20:08.
Injury check: Ok while running.
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Today was hard. 

I have various excuses reasons for it being hard (including the fact that in the last couple of days my hayfever has started kicking in and I could feel a familiar tightness in my chest when I started running, although it's nowhere as bad as it could be/has been before) but mostly I think it just wasn't my day. If you're a runner, you'll probably know that some days you simply can't seem to find your rhythm and the run doesn't go well, for no real reason. If you're not a runner - well, just take my word that this is a real thing. 

I also tried some interval training during this run, as suggested by one of my friends from work. The idea is to run fast (i.e. faster than your usual pace) for two minutes and then jog/walk for one, then repeat those three minutes as many times as you want/can; the theory is that this will help you go further and therefore build up your stamina. 

I tried this sort of run once before on the treadmill and I quite liked it, because it does actually make it easier to go the distance with breaks in between the fast sections. However, I found this much harder to do outside! Mainly because I don't know how fast to run in the 'fast' sections - how much speed can I put on briefly without tiring myself out and ruining the rest of the pattern? Well, I know the speed I can cope with: 10.5 km/h (11 km/h for the final push) - but I know that from the treadmill screen; I can't judge it yet on my own. 
I also still find it harder to run to a time rather than a distance; I don't really know why I see those two end goals as so different, but I do. 

Also, although The Knee behaved itself during the run today, once I'd stopped (and collapsed ungracefully on the ground) it suddenly decided to develop a really painful ache. Don't know what to do about that except try to keep up the strengthening exercises and see how it goes...

FYI, my running work friend also recommends doing burst sprint training up a hill, but, let's face it - I'm unlikely to ever do that.