You need to be working anaerobically for this to kick in as it uses another energy system, which name escapes me.
What with the cold weather, I'd say that it's more likely to be cramp, so try focusing on warming up. Never stretch your muscles on a cold body, always do a very light 5 minute jog, like you would do on a recovery run, followed up by static stretches. Only then I'd skip landing on the and do some dynamic stretching before setting off.
If you're sure it's the build up of lactic acid, you may want to try threshold (tempo) runs. Basically it's running at maixmum effort which your body can work for long periods, and if you're working on sprint training, just tone it down slightly. Not so much like Fartlek training.
The thought behind it is to increase your ability to cope with the build up of lactic acid as it can gradually increase the body's capacity to produce energy. Threshold running generally consists of a 10 minutes warm up, and 20-30 minutes of running just below your lactate (anaerobic) threshold, ending with a five minute cool down.
It's not your top speed, but comfortably hard. If you are out of breathe to talk, you're going too fast. It should be about 78-85% of your maximum heart rate which can be found by using a calculator if you search google.
Remember though, you haven't been jogging for a year. Sprinting is explosive bursts of pace, whereas jogging is endurance. Reading a book over the Christmas period, it highlighted Paula Radcliffe wouldn't come anywhere near Lauryn Williams in the 100 metres, yet Lauryn Williams wouldn't come anywhere near Paula Radcliffe in a marathon. 800m is relatively short for muscles to be easing up, I'd say just take it easier and gradually work harder. You can use
http://www.multimap.com for distances too. Don't increase your weekly mileage by more than 2 miles is the recommended distance if memory serves me correctly.
All the best with it, and let us know how you get on.
