Don't go wandering off into the countryside unprepared and alone- Make sure, if you haven't got a training buddy with you (and that is the ideal) that you have your fully charged mobile phone with you, that you have told someone of your route and expected timings and have arranged a cut off time at which point they will set about organising help in finding you.No matter how big and clever you are, nature is far bigger and cleverer and unfortunately men do die through accidents and exposure etc out on the hills.

Avoid moving on the hills in the dark or staying out in the hills overnight until you know what you are doing. Always take sufficient kit and supplies to cope with what can be fast changing conditions- as you are starting to train with weight in your bergan make that weight up with practical and useful kit.
Think safety at all times- you are no use to anyone injured or worse.
Get a map of the Brecon Beacons and find Pen-y-fan. Then go and look at it one weekend. Later in your training you will need to be able to run from the bottom at the Storey Arms Outdoor Pursuit centre up to the top and over, down the other side to the reservoir and back along the road with a fifty pound pack and webbing in four hours.
Go and walk it then you'll know what training you need!

Long drag is sixty klicks across the brecon beacons hitting most of the highest points in less than 20 hours with a fifty pound pack webbing and weapon on your own. So go out on the hills and get used to the terrain and work on your navigation skills.
Do NOT run with weight at this stage.
Ring your local unit and they will advise you

Take a look at http://www.ldwa.org.uk/

-Long Distance Walking Association web site.
They organise & provide details on events arount the country which are excellent for training i.e
Black Mountain Challenge ~30miler
Stretton Dale hike 50 miles.

The events don't cost a lot and proivide excellent opportunity to get out onto hills to practice map reading and hill walking/running - both in daylight and at night.
After doing a few events -try adding a little weight.

Also look into joining up with a local running/fell running club. The competition between members and races is a good motivator.