It's not your age, Whizzy. When 91-year-old men run marathons, NOTHING is allowed to be put down to your age. Especially as you're probably younger than I am...
A couple of thoughts. First, have you fully recovered from your recent bout of illness, or was that bout of illness your body's way of trying to tell you to ease off a bit?
Could you be trying to fit in too much? Four or five training days a week is quite enough, especially if one of them is a 10-13 mile run. If your training depends on work, does that mean that you also work long and erratic hours? Are you doing what I'm often tempted to do and making each training session a hefty one because you don't know when you'll get the opportunity to have another?
And, as Nic suggests, what about food? Are you as careful about fuelling up as you used to be, or are there dozens of things in your life that take priority over getting proper meals?
You will get back to peak fitness, but forcing in more and more training is only going to leave you burned-out or injured again. Have an easy couple of weeks (reward yourself for resisting the temptation to overdo things if that's what it takes) and do a bit of re-evaluation - look at all areas of your life, not just athletic training. Then choose a race a few months into the future as a target, and stick to the training schedules.
And keep us up to date with your progress!
Cheers, V-rap.