Hi Lots of people post about joining the TA with references to fitness...
You can turn in to your unit in any state of fitness provided you pass the medical and complete your paperwork you can begin recruit training.
HOWEVER! You would be well advised to start recreational running, there's a million books and websites on running, but if you follow a 10k program you'll be doing fine, enter a race! Get involved, join your local running club...
Most soldiers are runners in my exp. The fitter you are, the easier you will find recruit phase, the less likely you are to be injured, the more you can contribute to your units sporting endeavors!
Tom