I run slower in the mornings, but it's done and banked before a busy day can mess it up. However for interval training it has to be the evening, just can't get my legs to go quick enough in the morning. As for energy levels I find it gives me a boost.
I used to have a similar problem. I was simply taking to much on when running; energy drinks and water mainly. So now I take a bottle of water and sip it throughout an event. Also it may be your pacing/pyshcological when the end approaches you may be picking up the pace to much or even thinking yourself sick now.
If you don't do either of the above now you will be doing them, later in an epic hurry to be fit for London. In the meantime, stretch the calf and ice 3 times a day for 15 minutes. You can easily run through an Achilles problem to a point and long slow runs are comfortable with a sore tendon, however faster runs with a change of pace included can become a problem.
I'm just getting over an Achilles problem after over a year out because I ignored it initially.
The hell runs tend to bottle neck at various bogs, steep ascents, narrow paths etc. so the pace you run them at is often considerably slower than other races. You'll easily get round. When I did it I actually felt quite fresh at the end as I'd held back so much waiting for the 'hell' part to kick in but apart from getting filthy, wet and having to run up some steep (but short) hills its not to bad.