On the plus side, I've worked out why I've been getting slower and slower over the last month or two (and had such a problem at the Outlaw Half and then a truly ridiculous time in Liverpool!). On the down side, it's overtraining syndrome (or hopefully its more mild variant 'overreaching').
Overtraining syndrome occurs when, repeatedly, the body isn't able to recover adequately after a training session before the next (I think it would be more accurate to call it 'underrecovery syndrome'). It's important to realise that performance doesn't increase directly due to training (try doing a timed piece immediately after a hard interval session!) but due to overcompensation by the body during the recovery period. If the body isn't given chance to recover, you start your next session a little bit down on where you should be, and the next a little further, and so on until you find yourself spiralling into trouble.
The problem I've run into is that all stress slows recovery, not just training stress - so spending a week in bed comatose on tramadol after my latest face-surgery doesn't count as a rest. And naturally there's a tendency to get anxious about "time off training", and want to train more/harder, and then as the spiral starts to get anxious about declining performances, and train harder still. This is Bad.
Things to look out for (which I missed): declining performances, poor sleep, not being 'up for it' with workouts, failing to reach target training heart rate. Particularly I've found my "top end" has gone - long runs and rides seemed OK (albeit more tiring), but 400m running intervals were getting towards 3k pace!
The solution? Luckily (in some ways I guess) I've had this before when I decided Boat Race training "wasn't enough" and started running and cycling too. Last time a decent bit of time off worked wonders and later that same year I was posting some of my best rowing times; I'm hoping to go for the same again.
The long term solution? Listen to your body, and (for me) accept that having a major accident and repeated surgery is going to bugger up the year somewhat. Given that one of the inspirations of this triathlon project was my friend Matt and I discussing the merits of a "do what you feel like" flexible training program over a fixed, structured (and more time-intensive) program, I swayed dangerously into feeling I "needed" to do workout X or Y regardless of how knackered I was. Time to cut back! (and more time with the family is never a bad thing)