Posted: 29/12/2014 at 11:15
Whereabouts in Sussex are you? There are clubs across the county that do regular trail running so I'd agree with JR's advice, contact a local club and go along for an evening. I don't currently belong to a club myself, but most are pretty friendly.
Re training plans, I also agree with JR, except that I'd add in additional hill intervals if I really wanted to push my time. Also, if you can't get out for three days of the week, there are some excellent exercise programmes for trail runners, that involve about 30 minutes a session and can be done at home. Have a look round Youtube or similar. (Sorry, I have lost my links to the programmes I used, as they were on an old computer )
Good luck and enjoy!